Initialized flipper-plugin library

Summary: Set up an initial library which can (should) be used by plugins in the future.

Reviewed By: jknoxville

Differential Revision: D22019554

fbshipit-source-id: 502b14b34b2c9c117cea377ab6ebbf150e6faee9

desktop/scripts/start-dev-server.ts

@@ -199,7 +199,7 @@ async function startWatchChanges(io: socketIo.Server) {
     const watchman = new Watchman(path.resolve(__dirname, '..'));
     await watchman.initialize();
     await Promise.all(
-      ['app', 'pkg', 'doctor'].map((dir) =>
+      ['app', 'pkg', 'doctor', 'flipper-plugin'].map((dir) =>
         watchman.startWatchFiles(
           dir,
           () => {