Convert to TypeScript

This needs the usual "I know the `app` element exists" annotation, plus
dealing with a non-existing /etc/hostname (in which case the watch will
return `null`, and we shouldn't poke that into a `string` state).

src/index.tsx

@@ -0,0 +1,32 @@
+/*
+ * This file is part of Cockpit.
+ *
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * Cockpit is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or
+ * (at your option) any later version.
+ *
+ * Cockpit is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with Cockpit; If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import React from 'react';
+import { createRoot } from 'react-dom/client';
+
+import "cockpit-dark-theme";
+
+import { Application } from './app.jsx';
+
+import "patternfly/patternfly-5-cockpit.scss";
+import './app.scss';
+
+document.addEventListener("DOMContentLoaded", () => {
+    createRoot(document.getElementById("app")!).render(<Application />);
+});