[ENH]: Support avif as output format

### Problem

Since  v3.6.0, Matplotlib supports `webp` output (yay!), but I couldn't find a ticket tracking Avif support, so here's a feature request for `savefig` to support saving as `.avif`. 

*relevant links:*
- [CanIUse](https://caniuse.com/avif)
- [Pillow pull request](https://github.com/python-pillow/Pillow/pull/5201)


### Proposed solution

_No response_