Introduce Flipper Tic Tac Toe example

Summary:
This Diff introduces an example for how to develop a React Native pure JS plugin and will be used in the docs. See the attached project as demo. The RN sources for the plugin component are:

```
import React, {useState, useEffect} from "react";
import {
  StyleSheet,
  View,
  Text,
  Button,
} from 'react-native';

import {addPlugin} from "react-native-flipper";

const initialState = {
  cells: [" ", " ", " "," ", " ", " "," ", " ", " ",],
  turn: ' ',
  winner: ' ',
}

export default function FlipperTicTacToe() {
  const [status, setStatus] = useState("Waiting for Flipper Desktop Player...")
  const [gameState, setGameState] = useState(initialState);
  const [connection, setConnection] = useState(null);

  useEffect(() => {
    addPlugin({
      getId() {
        return 'ReactNativeTicTacToe';
      },
      onConnect(connection) {
        setStatus("Desktop player present");
        setConnection(connection);

        // listen to updates
        connection.receive('SetState', (gameState, responder) => {
          if (gameState.winner !== " ") {
            setStatus(`Winner is ${gameState.winner}! Waiting for a new game...`);
          } else {
            setStatus(gameState.turn === "X" ? "Your turn...": "Awaiting desktop players turn...");
          }
          setGameState(gameState);
          responder.success();
        })

        // request initial state
        connection.send('GetState');
      },
      onDisconnect() {
        setConnection(null);
        setStatus("Desktop player gone...");
      }
    })
  }, []);

  return (
    <View style={styles.container}>
      <Text style={styles.title}>Flipper Tic-Tac-Toe</Text>
      <Text>{status}</Text>
      <View style={styles.board}>
        {gameState.cells.map((state, idx) =>
          <View
            key={idx}
            style={styles.cell}>
            <Button
              title={state}
              disabled={gameState.turn !== 'X' || state !== ' '}
              onPress={() => {
                connection.send('XMove', { move: idx });
              }}
            />
          </View>
        )}
      </View>
    </View>
  )
}

// Omitted styling

```

Reviewed By: passy

Differential Revision: D19410138

fbshipit-source-id: 93266a1ef7b86dcf043a744c3563dab0c585c8fd

src/plugins/rn-example-plugin/index.tsx

@@ -1,56 +0,0 @@
-/**
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @format
- */
-
-import {View, FlipperPlugin, Button} from 'flipper';
-import React from 'react';
-
-type State = {};
-
-type PersistedState = {
-  count: number;
-  last: any;
-};
-
-export default class RnExamplePlugin extends FlipperPlugin<
-  State,
-  any,
-  PersistedState
-> {
-  static defaultPersistedState = {count: 0, last: null};
-
-  static persistedStateReducer(
-    persistedState: PersistedState,
-    method: string,
-    payload: any,
-  ) {
-    return {
-      count: persistedState.count + 1,
-      last: payload,
-    };
-  }
-
-  state = {};
-
-  render() {
-    const {persistedState} = this.props;
-
-    return (
-      <View>
-        <Button
-          onClick={async () => {
-            const result = await this.client.call('FromDesktop', {test: 123});
-            window.alert(result.test);
-          }}>
-          Send message
-        </Button>
-        <pre>{JSON.stringify(persistedState, null, 2)}</pre>
-      </View>
-    );
-  }
-}

src/plugins/rn-tic-tac-toe/index.tsx

@@ -0,0 +1,190 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import React from 'react';
+import {
+  FlipperPlugin,
+  RoundedSection,
+  Button,
+  produce,
+  CenteredView,
+  Info,
+  colors,
+  styled,
+  FlexRow,
+  Text,
+  brandColors,
+} from 'flipper';
+import {Draft} from 'immer';
+
+type Player = ' ' | 'X' | 'O';
+
+type State = {
+  cells: readonly [
+    Player,
+    Player,
+    Player,
+    Player,
+    Player,
+    Player,
+    Player,
+    Player,
+    Player,
+  ];
+  winner: Player;
+  turn: 'X' | 'O';
+};
+
+function initialState(): State {
+  return {
+    // Cells
+    // 0 1 2
+    // 3 4 5
+    // 6 7 8
+    cells: [' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '] as const,
+    turn: Math.random() < 0.5 ? 'O' : 'X',
+    winner: ' ',
+  } as const;
+}
+
+const computeNextState = produce(
+  (draft: Draft<State>, cell: number, player: 'X' | 'O') => {
+    draft.cells[cell] = player;
+    draft.turn = player === 'X' ? 'O' : 'X';
+    draft.winner = computeWinner(draft.cells);
+  },
+);
+
+function computeWinner(c: State['cells']): Player {
+  // check the 2 diagonals
+  if ((c[0] === c[4] && c[0] === c[8]) || (c[2] === c[4] && c[2] === c[6])) {
+    return c[4];
+  }
+  for (let i = 0; i < 3; i++) {
+    // check vertical
+    if (c[i] === c[3 + i] && c[i] === c[6 + i]) {
+      return c[i];
+    }
+    // check horizontal
+    if (c[i * 3] === c[i * 3 + 1] && c[i * 3] === c[i * 3 + 2]) {
+      return c[i * 3];
+    }
+  }
+  return ' ';
+}
+
+export default class ReactNativeTicTacToe extends FlipperPlugin<
+  State,
+  any,
+  any
+> {
+  state = initialState();
+
+  componentDidMount() {
+    this.client.subscribe('XMove', ({move}: {move: number}) => {
+      this.makeMove('X', move);
+    });
+    this.client.subscribe('GetState', () => {
+      this.sendUpdate();
+    });
+    this.sendUpdate();
+  }
+
+  makeMove(player: 'X' | 'O', move: number) {
+    if (this.state.turn === player && this.state.cells[move] === ' ') {
+      this.setState(computeNextState(this.state, move, player), () =>
+        this.sendUpdate(),
+      );
+    }
+  }
+
+  sendUpdate() {
+    this.client.call('SetState', this.state);
+  }
+
+  handleCellClick(move: number) {
+    this.makeMove('O', move);
+  }
+
+  handleReset() {
+    this.setState(initialState(), () => this.sendUpdate());
+  }
+
+  render() {
+    const {winner, turn, cells} = this.state;
+    return (
+      <CenteredView>
+        <RoundedSection title="React Native Tic-Tac-Toe">
+          <Info type="info">
+            This plugin demonstrates how to create pure JavaScript Flipper
+            plugins for React Native. Find out how to create a similar plugin at{' '}
+            <a
+              href="https://fbflipper.com/docs/tutorial/intro.html"
+              target="blank">
+              fbflipper.com
+            </a>
+            .
+          </Info>
+          <Container>
+            <Text size={24}>Flipper Tic-Tac-Toe</Text>
+            <br />
+            <br />
+            <Text size={18}>
+              {winner !== ' '
+                ? `Winner! ${winner}`
+                : turn === 'O'
+                ? 'Your turn'
+                : 'Mobile players turn..'}
+            </Text>
+            <GameBoard>
+              {cells.map((c, idx) => (
+                <Cell
+                  key={idx}
+                  disabled={c !== ' ' || turn != 'O' || winner !== ' '}
+                  onClick={() => this.handleCellClick(idx)}>
+                  {c}
+                </Cell>
+              ))}
+            </GameBoard>
+            <Button onClick={() => this.handleReset()}>Start new game</Button>
+          </Container>
+        </RoundedSection>
+      </CenteredView>
+    );
+  }
+}
+
+const Container = styled('div')({
+  border: `4px solid ${brandColors.Flipper}`,
+  borderRadius: 4,
+  padding: 20,
+  marginTop: 20,
+});
+
+const GameBoard = styled(FlexRow)({
+  flexWrap: 'wrap',
+  justifyContent: 'space-between',
+  marginTop: 20,
+  marginBottom: 20,
+});
+
+const Cell = styled('button')({
+  padding: 20,
+  height: 80,
+  minWidth: 80,
+  fontSize: 24,
+  margin: 20,
+  flex: 0,
+  borderRadius: 4,
+  backgroundColor: colors.highlight,
+  color: 'white',
+  ':disabled': {
+    backgroundColor: colors.greyTint2,
+  },
+});

src/plugins/rn-tic-tac-toe/package.json

@@ -1,12 +1,12 @@
 {
-  "name": "ReactNativeExamplePlugin",
+  "name": "ReactNativeTicTacToe",
   "version": "1.0.0",
   "main": "index.tsx",
   "license": "MIT",
   "keywords": ["flipper-plugin"],
   "icon": "apps",
-  "title": "React Native Example Plugin",
+  "title": "React Native Tic Tac Toe",
-  "category": "Example Plugin",
+  "category": "Examples",
   "bugs": {
     "email": "mweststrate@fb.com"
   }