Req(?): opt-in AVIF thumbnail support

[Quackdoc]

I'm not sure how it want's to be handled, but compiling cosmic-files with cargo build --release --features "image/avif-decoder" will properly enable the avif support in image-rs. However there are multiple issues with this. It has limited support, it can break on some images like gbrp it will generate wrong colours, Only supports 8bit images, It also requires building against dav1d.

I don't think it should be a default feature, but it might be nice to be able to set it as an optional feature, or even simply advertise building with --features "image/avif-decoder" in the readme.md