Rewrite <LoadingIndicator/> as FC and TS (#25364)

app/javascript/mastodon/components/loading_indicator.tsx

@@ -0,0 +1,7 @@
+import { CircularProgress } from './circular_progress';
+
+export const LoadingIndicator: React.FC = () => (
+  <div className='loading-indicator'>
+    <CircularProgress size={50} strokeWidth={6} />
+  </div>
+);