[whatwg/fetch] Reconsider default Accept values for images (Issue #1740)

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

It seems that the default value for the Accept header was decided in [#43](https://github.com/whatwg/fetch/issues/43#issuecomment-97909717)
It's possible things have changed a bit since then.
I did a quick check of what are the default accept headers for images and icons and got this:

Chrome 
icon: `image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8`
image: `image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8`

Firefox:
icon: `image/avif,image/jxl,image/webp,*/*`
image: `image/avif,image/jxl,image/webp,*/*`

(Note that `image/avif` and `image/jxl` might not be present for all users)

Safari:
icon: `*/*`
image: `image/webp,image/avif,image/jxl,image/heic,image/heic-sequence,video/*;q=0.8,image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5`

@zcorpan suggested it might be interesting to see if we want to reconsider the default values.

https://github.com/whatwg/fetch/blob/8dd73dbecfefdbef8f432164fb3a5b9785f7f520/fetch.bs#L4391
> "image"
>    `image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5` 

It's especially interesting that Firefox and Chrome don't have `image/png` anymore.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1711622#c13 for initial report.



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