AVIF support: add AVIF format to picture element

[Tabrisrp]

Scope a solution ✅

Refactor the picture display class to handle the AVIF format in addition WebP:

• Move it outside of the WebP namespace
• Update the build_source_tag() method to add AVIF
• Update the get_images() method to add AVIF
• Update the process_image() method to add AVIF

Estimate the effort ✅

Effort [S]