Media: enable AVIF support.
Add support for uploading, editing and saving AVIF images when supported by the server.
Add 'image/avif' to supported mime types. Correctly identify AVIF images and sizes even when PHP doesn't support AVIF. Resize uploaded AVIF files (when supported) and use for front end markup.
Props adamsilverstein, lukefiretoss, ayeshrajans, navjotjsingh, Tyrannous, jb510, gregbenz, nickpagz, JavierCasares, mukesh27, yguyon, swissspidy.
Fixes #51228.