Use `event.key` instead of deprecated `event.which`

site/src/routes/repl/[id]/_components/AppControls/index.svelte

@@ -30,7 +30,7 @@
 	$: canSave = $session.user && gist && gist.owner === $session.user.uid;
 
 	function handleKeydown(event) {
-		if (event.which === 83 && (isMac ? event.metaKey : event.ctrlKey)) {
+		if (event.key === 's' && (isMac ? event.metaKey : event.ctrlKey)) {
 			event.preventDefault();
 			save();
 		}