BREAKING CHANGE
Since `next export` has been printing a deprecation warning since https://github.com/vercel/next.js/pull/47376, its safe to remove in semver-major.
The upgrade path is to simply add `output: 'export'` in `next.config.js` - everything will continue to work the same.
This config greatly improves the `next dev` experience today. And in the future, it will improve performance of `next build` because we no longer need to do two passes (build then export).
This fixes our check for fallback pages during `next export` as we are currently erroring even when the erroneous pages are not included in the `exportPathMap`. This also adds additional tests to certify the expected behavior for the error.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`
Fixes: https://github.com/vercel/next.js/issues/29135
