Upgrade to TypeScript 4.1

Summary:
allow-large-files

Upgrade to TypeScript 4.1

Reviewed By: nikoant

Differential Revision: D25122073

fbshipit-source-id: 7d2bc6f97595c825c9527b0c6ecab07698d5f981

desktop/app/src/server.tsx

@@ -157,7 +157,7 @@ class Server extends EventEmitter {
               'server',
             );
             server.emit('listening', port);
-            resolve(rsServer);
+            resolve(rsServer!);
           });
         return transportServer;
       };