Introduce showNotification API

Summary: Introduced `showNotifcation` to the Sandy API.

Reviewed By: jknoxville

Differential Revision: D27012001

fbshipit-source-id: d3f237910a478400b0f925f0362af485c96072bb

desktop/flipper-plugin/src/index.ts

@@ -46,6 +46,7 @@ export {
   buildInMenuEntries as _buildInMenuEntries,
   DefaultKeyboardAction,
 } from './plugin/MenuEntry';
+export {Notification} from './plugin/Notification';
 
 export {theme} from './ui/theme';
 export {Layout} from './ui/Layout';