feat(ags): init gtk4 migration attempt

modules/ags/gtk4/app.ts

@@ -0,0 +1,15 @@
+import { App } from 'astal/gtk4';
+
+import style from './style.scss';
+
+import Bar from './widget/Bar';
+
+
+App.start({
+    css: style,
+    instanceName: 'gtk4',
+
+    main() {
+        Bar();
+    },
+});