Force Vercel to create separate cache entries for RSC & HTML responses

By specifying two different destinations (`/` and `/index`) that point to the same edge function we can apparently separate the RSC response cache from the HTML response cache (based on the `accept` header).

Motivated by https://github.com/vercel/vercel/blob/732ac2072cd1c09e383d7da2f671c3bb430c8a8a/packages/next/src/server-build.ts#L1466-L1497

apps/vercel-app/src/config.json

@@ -14,11 +14,16 @@
       "src": "/_vercel/insights/(.*)",
       "dest": "https://vitals.vercel-insights.com/v1/$1"
     },
+    {
+      "src": "/.*",
+      "dest": "/index",
+      "has": [{"type": "header", "key": "accept", "value": "text/x-component"}],
+      "headers": {"vary": "accept"}
+    },
     {
       "src": "/.*",
       "dest": "/",
-      "headers": {"vary": "accept"},
-      "override": true
+      "headers": {"vary": "accept"}
     }
   ]
 }