This is a follow-up to https://github.com/vercel/next.js/pull/28227 to ensure `_app` and `_document` HMR correctly when you start the dev server and then add `_app` and `_document`.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`
x-ref: https://github.com/vercel/next.js/issues/27888