Track UI launches

Summary: This is currently done by querying the 'console-log' event. Instead, make it type safe and official.

Reviewed By: antonk52

Differential Revision: D50927713

fbshipit-source-id: 780c0f158bf8bdaa6d009035a0cee62c7828f479

desktop/flipper-server-core/src/tracker.tsx

@@ -47,6 +47,7 @@ type TrackerEvents = {
     error?: string;
   };
   'server-socket-already-in-use': {};
+  'server-open-ui': {browser: boolean; hasToken: boolean};
   'browser-connection-created': {
     successful: boolean;
     timeMS: number;