Revert "Revert "Refactor Server Router" (#40328)" (#40333)

This unreverts #40328 as it wasn't
the cause for #40241 like
initially suspected and the actual fix has been included here as well
with regression test.

Fixes: #40241

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/server/base-server.ts

@@ -959,7 +959,11 @@ export default abstract class Server<ServerOptions extends Options = Options> {
 
     // Toggle whether or not this is a Data request
     const isDataReq =
-      !!(query.__nextDataReq || req.headers['x-nextjs-data']) &&
+      !!(
+        query.__nextDataReq ||
+        (req.headers['x-nextjs-data'] &&
+          (this.serverOptions as any).webServerConfig)
+      ) &&
       (isSSG || hasServerProps || isServerComponent)
 
     delete query.__nextDataReq

packages/next/server/dev/next-dev-server.ts

@@ -221,7 +221,6 @@ export default class DevServer extends Server {
       for (const path in exportPathMap) {
         const { page, query = {} } = exportPathMap[path]
 
-        // We use unshift so that we're sure the routes is defined before Next's default routes
         this.router.addFsRoute({
           match: getPathMatch(path),
           type: 'route',