Fix basic-app benchmark application (#60842)

Fairly basic PR that just fixes the `basic-app` benchmark application.
The api route was throwing an error since it wasn't returning a
`Response`, and renamed some `pages/` directory routes so they become
available.

bench/basic-app/app/api/app/route.js

@@ -1,5 +1,5 @@
 export function GET() {
-  return { name: 'John Doe' }
+  return Response.json({ name: 'John Doe' })
 }
 
 export const dynamic = 'force-dynamic'