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
Since we are no longer accepting new built-in loaders, users may wish to use a different cloud provider.
So this PR renames `dangerously-unoptimized` to `custom` to handle this case as well as the intention of `next export`.
If the user doesn't add a `loader` prop, we throw an error.
If the user adds a `loader` prop but it doesn't return the width, we print a warning.
- Follow up to #26847
- Fixes#21079
- Fixes#19612
- Related to #26850
