Allow plugins to use css

Summary:
Flipper plugins fail when importing css from third-party dependencies. This diff tries to fix that.

Effectively, the plugin can import the css and export it when is bundled.

When we load the plugin, we check if there's a css file for it. If there's one, we return it and try to use it.

Reviewed By: aigoncharov

Differential Revision: D40758178

fbshipit-source-id: e53afffcc481504905d5eeb1aea1f9114ee2a86b

desktop/plugins/public/ui-debugger/index.tsx

@@ -9,6 +9,7 @@
 
 import {PluginClient, createState, createDataSource} from 'flipper-plugin';
 import {Events, Id, PerfStatsEvent, Snapshot, TreeState, UINode} from './types';
+import './node_modules/react-complex-tree/lib/style.css';
 
 export function plugin(client: PluginClient<Events>) {
   const rootId = createState<Id | undefined>(undefined);