fixes#26607
This change makes it so the image loader plugin only emits a file while processing an image import for the client. The final generated image URL was already the same in SSR and CSR anyway, so this change doesn't have any functional impact.
I also changed the name of the static page in the image component tests, since it was causing some conflicts with the static assets folder.
There are strict conditions for using `placeholder=blur` documented in #25949 but this will give the user a better understanding during `next dev` and links to the error.
- Error when `placeholder=blur` and no `blurDataURL`
- The Error for small images with `placeholder=blur` has been changed to a warning
- Added support for blurring a webp image
- Added error page linking to relevant docs
