AVIF conversion with WEBP fallback

Problem/Motivation

Not all servers support converting to AVIF, so we can't use it as a default format for image styles in install profiles.

Steps to reproduce

1. Have a server that doesn't support AVIF
2. Use the Convert image style plugin configured to use AVIF
3. Create an article with an image, the image will not display

Proposed resolution

As per #3372932-40: [Meta] High-performance images (nearly) out of the box we may need a new Image Effect to convert to AVIF if it's supported, with a fallback to WEBP if it's not. Possibly this could be generic for other formats too, but for now only AVIF has explicit requirements.

Remaining tasks

1. Decide if we want to do this
2. Review the approach in the MR
3. Make the IS and CR sparkle

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet