Add headless-demo plugin and its usage example

Summary:
Next steps:
1. Refactor it TS for consistency
2. Remove it in favor of tic-tac-toe integration

Reviewed By: mweststrate

Differential Revision: D36102002

fbshipit-source-id: 7dc930f67bed636159a2ec433d7405ab6ee09f97

desktop/plugins/public/headless-demo/index.tsx

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and 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 {createState, Layout, FlipperPluginInstance} from 'flipper-plugin';
+
+export function API(
+  pluginInstance: FlipperPluginInstance<typeof devicePlugin>,
+) {
+  return {
+    increment: pluginInstance.increment,
+  };
+}
+
+export function devicePlugin() {
+  const data = createState(0);
+
+  const increment = (step: number = 1) => {
+    const newVal = data.get() + step;
+    data.set(newVal);
+    return newVal;
+  };
+
+  return {increment};
+}
+
+export function Component() {
+  return (
+    <Layout.ScrollContainer>
+      I am a new shiny headless plugin
+    </Layout.ScrollContainer>
+  );
+}