Add AVIF thumbnail support #210

Using <object> would allow a fallback image to be specified for browsers which do not support AVIF.

For a bit more context, the main problem area for this right now would be adequate AVIF encoding support in major distros. FFmpeg can't yet understand AVIF files. There's a libavif library (with associated utilities that TinyIB could make use of directly) which is packaged by Debian and Ubuntu, but it's currently only compiled with decoding support because the only AV1 encoder library in their repos is libaom. The packaged version of libaom is so old that libavif doesn't support it, and there hasn't been movement on it since Debian is in freeze right now.

As soon as the libaom maintainer updates it post-freeze, or a different encoder like rav1e is packaged, I expect encoding functionality in libavif will be toggled on. The first stable distribution releases that would be available in would likely be Debian 12 and Ubuntu 22.04. In the meantime, it'd still be useful for people using Fedora, Arch, Debian Testing, or others that are more bleeding-edge.