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/flipper-common/src/server-types.tsx

@@ -176,6 +176,10 @@ export interface DeviceDebugData {
   data: (DeviceDebugFile | DeviceDebugCommand)[];
 }
 
+export interface PluginSource {
+  js: string;
+  css?: string;
+}
 export type FlipperServerCommands = {
   'get-server-state': () => Promise<{
     state: FlipperServerState;
@@ -275,7 +279,7 @@ export type FlipperServerCommands = {
   'plugin-start-download': (
     plugin: DownloadablePluginDetails,
   ) => Promise<InstalledPluginDetails>;
-  'plugin-source': (path: string) => Promise<string>;
+  'plugin-source': (path: string) => Promise<PluginSource>;
   'plugins-install-from-marketplace': (
     name: string,
   ) => Promise<InstalledPluginDetails>;