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/scripts/jest-setup-after.tsx

@@ -190,7 +190,7 @@ function createStubRenderHost(): RenderHost {
     },
     flipperServer: TestUtils.createFlipperServerMock(),
     async requirePlugin(path: string) {
-      return require(path);
+      return {plugin: require(path)};
     },
     getStaticResourceUrl(relativePath): string {
       return 'file://' + resolve(rootPath, 'static', relativePath);