Add .avif to default cached extensions

.avif image files should be added to the default cached extensions like other image files.

The AVIF image format is now supported by Chrome, behind a flag for Firefox, and is in development for Safari.

Cloudflare Image Resizing now supports AVIF, but .avif files are not yet included in the file extensions that are cached by default.

As a workaround I created a “Cache Level: Cache Everything” page rule, but I only need this to ensure .avif images are cached and could remove it if they were cached by default like other image files.