Add initial desktop files

Summary:
This is just an initial setup from Flipper. I just ran `scarf flipper-plugin`, followed the iOS path, removed the native files (as we will use the JS infra directly later on) and submitted. :)

I followed this documentation/tutorial: https://www.internalfb.com/intern/wiki/Flipper/create-new-plugin/

(Followed iOS path, then removed native code as not needed)

Differential Revision: D22754500

fbshipit-source-id: 39130b4ee17c6bc72990eec34c466cfa7a650d17

desktop/static/icons.json

@@ -433,5 +433,8 @@
   ],
   "caution-circle": [
     12
+  ],
+  "megaphone": [
+    12
   ]
 }