Feature Description

Inform users when server lacks AVIF support, similar to the WebP status information we added in #130.

Since not every server environment supports translating JPEG uploads to AVIF format, it's worth indicating this to site administrators in an easy-to-understand way. While we already have some basic PHP extension information about image capabilities in the Site Health info tab, it may be worth adding something explicitly about AVIF in the Site Health "Status" tab.