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/app.tsx

@@ -30,7 +30,7 @@ export const Application = () => {
 
     useEffect(() => {
         const hostname = cockpit.file('/etc/hostname');
-        hostname.watch(content => setHostname(content.trim()));
+        hostname.watch(content => setHostname(content?.trim() ?? ""));
         return hostname.close;
     });
 

src/index.tsx

@@ -28,5 +28,5 @@ import "patternfly/patternfly-5-cockpit.scss";
 import './app.scss';
 
 document.addEventListener("DOMContentLoaded", () => {
-    createRoot(document.getElementById("app")).render(<Application />);
+    createRoot(document.getElementById("app")!).render(<Application />);
 });