Modern Image Formats: add AVIF output support

[adamsilverstein]

Summary

Add AVIF support for Modern Image Formats plugin.

Fixes #1151

Relevant technical choices

• When the server supports AVIF, add a dropdown under settings->media where users can choose between AVIF and WebP as the output format
• Slightly rename the field that enables generating JPEG images alongside the modern output format (so it isn't just about WebP plug JPEG).
  • note: I left the option name unchanged for backwards compatibility.
• I removed the CSS class that were hiding part of the settings UI, that felt hacky and made adding the new field more challenging.

Screenshots

Media settings

The new settings fields on media, the new dropdown is only shown if AVIF is supported on the server:

note: the default selection is AVIF when available.

Uploads

• Note: activate twenty-twentyone theme

Output set to AVIF

Output set to WebP

With "Also Output JPEGs" checked

AVIF

WebP

Note: there appears to be a bug when the additional JPEG option is selected. I will double check, I believe this problem is already present in trunk. Looking carefully at the final two images where this setting is enabled, the AVIF/WebP images are only generated in the default (small/medium/medium/large/large and -scaled) variations it looks like custom sizes and maybe the high dpi images are missing in the modern format. I can create a separate issue to investigate this further.

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[adamsilverstein]

It would be good to add some tests here. I was expecting existing tests to fail, since I changed the default output to AVIF - maybe the test runners don't have AVIF support.

[westonruter]

Last minor requested changes!

[joemcgill]

This is testing well for me. Pre-approving pending @weston's feedback

[westonruter]

🎉

[adamsilverstein]

🎉

"At least 1 approving review is required to merge this pull request."

[adamsilverstein]

🎉

"At least 1 approving review is required to merge this pull request."

#1257 should let you approve this one @westonruter