[Next.js] Investigate whether source maps are working for files in route groups

[lforst]

Description

https://nextjs.org/docs/app/building-your-application/routing/route-groups

I have a vague recollection of seeing an issue where sourcemaps didn't work for route groups for whatever reason. We should check that because route groups are used quite commonly.

[lforst]

Seems to work fine