fix: add missing sign out page route for next.js app dir sample (#733)

The `/api/logto/sign-out` route is missing, this commit fix it, otherwise it
will lead to a 404 not found.

packages/next-app-dir-sample/app/api/logto/sign-out/route.ts

@@ -0,0 +1,9 @@
+import { type NextRequest } from 'next/server';
+
+import { logtoClient } from '../../../../libraries/logto-edge';
+
+export const runtime = 'edge';
+
+export async function GET(request: NextRequest) {
+ return logtoClient.handleSignOut()(request);
+}