Rewrite <Skeleton/> as FC and TS (#25055)

app/javascript/mastodon/components/skeleton.tsx

@@ -0,0 +1,12 @@
+import React from 'react';
+
+interface Props {
+  width?: number | string;
+  height?: number | string;
+}
+
+export const Skeleton: React.FC<Props> = ({ width, height }) => (
+  <span className='skeleton' style={{ width, height }}>
+    &zwnj;
+  </span>
+);