AVIF support

This will be dependent on https://github.com/lovell/sharp/issues/2289

Sharp can already support AVIF format but requires a specially built libvips binary which we might be able to achieve but better to wait for the official support IMO.

For an overview and breakdown of AVIF and the kind of compression it can achieve see this post by Jake Archibald:

https://jakearchibald.com/2020/avif-has-landed/

Browser support may look the same as how we do webp detection right now. Alternatively in terms of implementation we could look to using `` at the WordPress level.

Start by reviewing the dependency in sharp issue 2289 and the existing WebP browser-detection behavior. Compare the proposed WordPress-level picture approach with that detection path; done means AVIF support is implemented once the required libvips support is available and the browser behavior is covered.

Written by the indexing model from the issue text.