Add ability to dynamically extend the layout inspector sidebar

Summary: This adds the ability to extend the layout inspector's sidebar via SidebarExtensions

Reviewed By: sjkirby

Differential Revision: D8677332

fbshipit-source-id: 1880d5d2185306290a278e8a48e965ad94b98cc0

src/fb-stubs/LayoutInspectorSidebarExtensions.js

@@ -0,0 +1,7 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+export default [];