Add screensharing behind a feature flag

ts/windows/screenShare.ts

@@ -0,0 +1,11 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// This needs to use window.React & window.ReactDOM since it's
+// not commonJS compatible.
+window.registerScreenShareControllerRenderer((Component, props) => {
+  window.ReactDOM.render(
+    window.React.createElement(Component, props),
+    document.getElementById('app')
+  );
+});