Feature request: support for AVIF image format

Hi,

I love to see support for AVIF image format. AVIF was [released ](https://en.wikipedia.org/wiki/AVIF)on 19 february 2019 an is bassed upon the AV1 royalty-free video coding format. Avif generally has better compression than WebP. [This](https://netflixtechblog.com/avif-for-next-generation-image-coding-b1d75675fe4) is wat netflix says about AVIF.

For example: Purposeday.nl can save about 41.6% of image data if AVIF was used instead of WebP. [source](https://webspeedtest.cloudinary.com/)

Support:
Chrome fully supports AVIF image format. Check it [here](https://caniuse.com/avif)

The issue does not identify any files, tests, or entry points. Start by locating the plugin's image-format and cache-processing code, then establish the required AVIF behavior and tests; done means the plugin supports AVIF without regressing existing image formats.

Written by the indexing model from the issue text.