feature request: onClose call when reloading the app

```diff
    server.watcher.on('change', debounce(async () => {
+      await (app.onClose?.() ?? Promise.resolve())
      await _loadApp(config);
    }, debounceDelayMs));
```

would be nice to have this. simple and non invasive. if the app exposes a `onClose` method, it's called an awaited to allow destruction before reload. helps to sever persistent connections to 3rd party services.