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
This adds some missing fields to the `NextConfig` type we expose under `next` and also adds a `NextConfigComplete` internal type that allows us to not treat all fields as optional like you would when using the type in `next.config.js`.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes
