This PR does a few things:
- Moves `<noscript>` usage below the blur image since so that the `<noscript>` image renders on top of the blur image
- Remove the `isVisible` check for `<noscript>` since we can't rely on client-side JS
- Add `loading=lazy` to the `<noscript>` image to take advantage of native lazy loading (can't rely on JS lazy loading)
Fixes#28251
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
