fix: add missing middleware runtime file (#762)

Author: ascorbic

demo/pages/middle/_middleware.js

@@ -0,0 +1,3 @@
+export function middleware(req, ev) {
+    return new Response('Hello, middleware!');
+}

demo/pages/middle/index.js

@@ -0,0 +1,10 @@
+const Page = () => {
+
+  return (
+    <div>
+      <p>No middleware</p>
+    </div>
+  )
+}
+
+export default Page