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/app/src/dispatcher/iOSDevice.tsx

@@ -11,7 +11,7 @@ import {ChildProcess} from 'child_process';
 import {Store} from '../reducers/index';
 import {setXcodeDetected} from '../reducers/application';
 import {Logger} from '../fb-interfaces/Logger';
-import {DeviceType} from 'flipper-plugin';
+import type {DeviceType} from 'flipper-plugin';
 import {promisify} from 'util';
 import path from 'path';
 import child_process from 'child_process';