ui-next: add sample plugin

packages/ui-next/src/app.tsx

@@ -1,6 +1,6 @@
 import { Link } from './components/link';
 import { usePageData } from './context/page-data';
-import { before, defineSlot } from './registry';
+import { defineSlot } from './registry';
 
 const AppInner = defineSlot('page:app', () => {
   const data = usePageData();
@@ -15,13 +15,4 @@ const AppInner = defineSlot('page:app', () => {
   );
 });
 
-before('page:app', () => {
-  console.log('before app');
-  return (
-    <>
-      <div>before app</div>
-    </>
-  );
-});
-
 export default AppInner;