Add JPEG XL (image/jxl)

[jonsneyers]

JPEG XL, image/jxl, ISO/IEC 18181.
More information can be found here:

• https://jpeg.org/jpegxl
• https://jpegxl.info

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This change is 

[halostatue]

image/jxl is not a registered type. ISO-IEC JTC1 has a number of items in image/*, but not jxl: https://www.iana.org/assignments/media-types/media-types.xhtml#image

Name	Template	Reference
avci	image/avci	[ISO-IEC_JTC1][David_Singer]
avcs	image/avcs	[ISO-IEC_JTC1][David_Singer]
heic	image/heic	[ISO-IEC_JTC1][David_Singer]
heic-sequence	image/heic-sequence	[ISO-IEC_JTC1][David_Singer]
heif	image/heif	[ISO-IEC_JTC1][David_Singer]
heif-sequence	image/heif-sequence	[ISO-IEC_JTC1][David_Singer]
hej2k	image/hej2k	[ISO-IEC_JTC1][ITU-T]
hsj2	image/hsj2	[ISO-IEC_JTC1][ITU-T]
jph	image/jph	[ISO-IEC_JTC1][ITU-T]
jphc	image/jphc	[ISO-IEC_JTC1][ITU-T]
jxr	image/jxr	[ISO-IEC_JTC1][ITU-T]
jxrA	image/jxrA	[ISO-IEC_JTC1][ITU-T]
jxrS	image/jxrS	[ISO-IEC_JTC1][ITU-T]
jxs	image/jxs	[ISO-IEC_JTC1]
jxsc	image/jxsc	[ISO-IEC_JTC1]
jxsi	image/jxsi	[ISO-IEC_JTC1]
jxss	image/jxss	[ISO-IEC_JTC1]

We can add this, but it should not be marked as registered until it’s on the IANA website.

[halostatue]

I have, ahem, provisional support for this in mime-types/ruby-mime-types#157 and #45. The way that I’m working with the provisional types makes it difficult to have extended information like extensions, but it means that I won’t have an issue where a type shows up twice (e.g., for image/jxl, once in types/image.yml and once in types/provisional-standard-types.yml). It’s not impossible (types/provisional-standard-types.yml can be edited like anything else, but when a type is removed, the extra data will not automatically carry over).

[halostatue]

This has been partially supported and merged via the support for provisional types and via 1876f76. Please note that the only thing not included in the provisional types is the extension jxl; this is unavoidable (the information is not currently in the IANA data) and for policy reasons explained in Contributing.md, pull requests to update provisional types with extensions will not be supported. Thanks for your contribution!