next.js: add option to use bundled runtime (#10485)

This PR adds an environment variable that should allow us to test the bundled version for Next.js on Vercel, see https://github.com/vercel/next.js/pull/52997 for reference.

The changes include:
- a new environment variable `VERCEL_NEXT_BUNDLED_SERVER`
- some logic changes that will put app route handlers into their own lambda groups
- extra logic to require early the rendering runtimes (see PR above for details)

packages/next/src/utils.ts

@@ -1391,8 +1391,10 @@ export type LambdaGroup = {
   memory?: number;
   maxDuration?: number;
   isAppRouter?: boolean;
+  isAppRouteHandler?: boolean;
   isStreaming?: boolean;
   isPrerenders?: boolean;
+  isPages?: boolean;
   isApiLambda: boolean;
   pseudoLayer: PseudoLayer;
   pseudoLayerBytes: number;