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/flipper-plugin/src/index.ts

@@ -0,0 +1,12 @@
+/**
+ * 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
+ */
+
+export function hello() {
+  return 'universe  ';
+}