Convert plugin.js to plugin.tsx

Summary:
* Deletes plugin.js
* Adds plugin.tsx
* Adds plugin flow-typed module that has the old flow types

Reviewed By: passy

Differential Revision: D16668067

fbshipit-source-id: b2f0ce47c4cf7125b4e352821e921b97675d12a9

src/plugins/example/index.js

@@ -7,7 +7,7 @@
  */
 
 import {Button, Input, FlipperPlugin, FlexColumn, styled, Text} from 'flipper';
-import type {Notification} from '../../plugin';
+import type {Notification} from '../../plugin.tsx';
 type DisplayMessageResponse = {
   greeting: string,
 };