[whatwg/fetch] Accept header for `image` destination should be updated for modern image formats (Issue #1905)

Lubrsi created an issue (whatwg/fetch#1905)

### What is the issue with the Fetch Standard?

The currently specified Accept header for the `image` destination is: `image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5`. However, this does not include modern formats such as AVIF and WebP.
For example, Chrome, Firefox and Safari now intersect with adding `image/avif` and `image/webp` to the Accept header.

There are live websites depending on these. For example, <https://tvtropes.org/> relies on this for Cloudflares image re-encoding to create WebPs, which preserves the transparency of the PNGs they use. Without WebP in the Accept header, it re-encodes to JPEG, which doesn't preserve transparency.

Without WebP in the Accept header:

<img width="823" height="220" alt="Image" src="https://github.com/user-attachments/assets/79152e76-1307-40e6-b1ed-85263729122a" />

With WebP in the Accept header:

<img width="808" height="214" alt="Image" src="https://github.com/user-attachments/assets/255b4817-cffb-4310-bb4b-d634b80d8ad1" />

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