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/plugins/seamammals/src/index.tsx

@@ -49,6 +49,22 @@ export function plugin(client: FlipperClient<Events, {}>) {
   const rows = createState<PersistedState>({}, {persist: 'rows'});
   const selectedID = createState<string | null>(null, {persist: 'selection'});
 
+  client.addMenuEntry(
+    {
+      label: 'Reset Selection',
+      topLevelMenu: 'Edit',
+      handler: () => {
+        selectedID.set(null);
+      },
+    },
+    {
+      action: 'createPaste',
+      handler: () => {
+        console.log('creating paste');
+      },
+    },
+  );
+
   client.onMessage('newRow', (row) => {
     rows.update((draft) => {
       draft[row.id] = row;