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
