Add image/jxl

JPEG XL (.jxl) is the next-generation, general-purpose, royalty-free image compression codec by the JPEG committee.

Chromium and Firefox already have preliminary support for JPEG XL.
It is included in the official repositories in FreeBSD and OpenMandriva.

See also:
https://jpegxl.info/
https://en.wikipedia.org/wiki/JPEG_XL
https://www.iana.org/assignments/provisional-standard-media-types/provisional-standard-media-types.xhtml
https://gitlab.com/wg1/jpeg-xl/-/blob/master/plugins/mime/image-jxl.xml