[macOS Monterey and Big Sur] Enable AVIF image decoding using libavif and dav1d
<rdar://problem/101481679>
Pull request: https://github.com/WebKit/WebKit/pull/5690
Committed 255984@main (5300e66a81df): <https://commits.webkit.org/255984@main> Reviewed commits have been landed. Closing PR #5690 and removing active labels.