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/utils/flipperLibImplementation.tsx

@@ -0,0 +1,38 @@
+/**
+ * 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
+ */
+
+import type {FlipperLib} from 'flipper-plugin';
+import type {Logger} from '../fb-interfaces/Logger';
+import type {Store} from '../reducers';
+
+let flipperLibInstance: FlipperLib | undefined;
+
+export function initializeFlipperLibImplementation(
+  _store: Store,
+  _logger: Logger,
+) {
+  // late require to avoid cyclic dependency
+  const {addSandyPluginEntries} = require('../MenuBar');
+  flipperLibInstance = {
+    enableMenuEntries(entries) {
+      addSandyPluginEntries(entries);
+    },
+  };
+}
+
+export function getFlipperLibImplementation(): FlipperLib {
+  if (!flipperLibInstance) {
+    throw new Error('Flipper lib not instantiated');
+  }
+  return flipperLibInstance;
+}
+
+export function setFlipperLibImplementation(impl: FlipperLib) {
+  flipperLibInstance = impl;
+}