Implement JS flipper client

Summary:
Standardize WS implementation for JS environments.

Why do we need a separate server implementation for browsers?
Browser targets cannot authenticate via the default certificate exchange flow. We need a dedicated client for them that works over an insecure channel (without the cert exchange).

Major changes:
1. Renamed `flipper-js-client-sdk` to `js-flipper` for consistency with `react-native-flipper`
2. Updated `js-flipper` implementation to match our other existing clients

Documentation will be updated in a separate subsequent PR.

https://fb.quip.com/2mboA0xbgoxl

Reviewed By: mweststrate

Differential Revision: D31688105

fbshipit-source-id: 418aa80e0fd86361c089cf54b0d44a8b4f748efa

js/js-flipper/src/plugin.ts

@@ -0,0 +1,68 @@
+/**
+ * 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
+ */
+
+export type FlipperPluginReceiverRes =
+  | object
+  | string
+  | number
+  | boolean
+  | null
+  | undefined
+  | void;
+export type FlipperPluginReceiver = (
+  data: any,
+) => FlipperPluginReceiverRes | Promise<FlipperPluginReceiverRes>;
+export interface FlipperPluginConnection {
+  /**
+   * Send an `execute` message to Flipper.
+   * Here is what client sends over the wire:
+   * { method: 'execute', params: { api: pluginID, method, params } }
+   *
+   * @param method Method name that needs to be executed by Flipper
+   * @param params Any extra params required for the method execution
+   */
+  send(method: string, params?: unknown): void;
+  /**
+   * Listen to messages for the method provided and execute a callback when one arrives.
+   * Send response back to Flipper.
+   * Read more about responses at https://fbflipper.com/docs/extending/new-clients#responding-to-messages
+   *
+   * @param method Method name that Flipper sent to the client
+   * @param receiver A callback executed by the client when a message with the specified method arrives.
+   * If this callback throws or returns a rejected Promise, client send an error message to Flipper.
+   * If this callback returns any value (even undefined) synchronously, client sends it as a success message to Flipper.
+   * If this callback returns a Promise, clients sends the value it is resolved with as a success message to Flipper.
+   */
+  receive(method: string, receiver: FlipperPluginReceiver): void;
+}
+export interface FlipperPlugin {
+  /**
+   * @return The id of this plugin. This is the namespace which Flipper desktop plugins will call
+   *     methods on to route them to your plugin. This should match the id specified in your React
+   *     plugin.
+   */
+  getId(): string;
+
+  /**
+   * Called when a connection has been established. The connection passed to this method is valid
+   * until {@link FlipperPlugin#onDisconnect()} is called.
+   */
+  onConnect(connection: FlipperPluginConnection): void;
+
+  /**
+   * Called when the connection passed to `FlipperPlugin#onConnect(FlipperConnection)` is no
+   * longer valid. Do not try to use the connection in or after this method has been called.
+   */
+  onDisconnect(): void;
+
+  /**
+   * Returns true if the plugin is meant to be run in background too, otherwise it returns false.
+   */
+  runInBackground?(): boolean;
+}