feat: add rewrites for API endpoint in Vercel configuration

Author: kentaro-m

vercel.json

@@ -6,5 +6,8 @@
   },
   "env": {
     "NODEJS_HELPERS": "0"
-  }
+  },
+  "rewrites": [
+    { "source": "/", "destination": "/api/github/webhooks" }
+  ]
 }