Introduce menu entry support

Summary:
[interesting] since it shows how Flipper APIs are exposed through sandy. However, the next diff is a much simpler example of that

This diff adds support for adding menu entries for sandy plugin (renamed keyboard actions to menus, as it always creates a menu entry, but not necessarily a keyboard shortcut)

```

  client.addMenuEntry(
    // custom entry
    {
      label: 'Reset Selection',
      topLevelMenu: 'Edit',
      handler: () => {
        selectedID.set(null);
      },
    },
    // based on built-in action (sets standard label, shortcut)
    {
      action: 'createPaste',
      handler: () => {
        console.log('creating paste');
      },
    },
  );
```

Most of this diff is introducing the concept of FlipperUtils, a set of static Flipper methods (not related to a device or client) that can be used from Sandy. This will for example be used to implement things as `createPaste` as well

Reviewed By: nikoant

Differential Revision: D22766990

fbshipit-source-id: ce90af3b700e6c3d9a779a3bab4673ba356f3933

desktop/flipper-plugin/src/index.ts

@@ -7,6 +7,7 @@
  * @format
  */
 
+import './plugin/PluginBase';
 import * as TestUtilites from './test-utils/test-utils';
 
 export {
@@ -26,6 +27,12 @@ export {SandyPluginDefinition} from './plugin/SandyPluginDefinition';
 export {SandyPluginRenderer} from './plugin/PluginRenderer';
 export {SandyPluginContext, usePlugin} from './plugin/PluginContext';
 export {createState, useValue, Atom} from './state/atom';
+export {FlipperLib} from './plugin/FlipperLib';
+export {
+  MenuEntry,
+  NormalizedMenuEntry,
+  buildInMenuEntries,
+} from './plugin/MenuEntry';
 
 // It's not ideal that this exists in flipper-plugin sources directly,
 // but is the least pain for plugin authors.