Register MYST as an official markdown variant with IANA

[stevejpurves]

IANA accepted an official registration for the text/markdown mimetype here: https://www.iana.org/assignments/media-types/text/markdown

Section 6.1 of the RFC specified a IANA based registry for markdown variants that is established here: https://www.iana.org/assignments/markdown-variants/markdown-variants.xhtml

The list of variants registered is woefully lacking but the registration mechanism allow formalize which mimetype variant parameter should be used to identify a flavor of markdown.

i.e. text/markdown;variant=MYST or text/markdown;variant=GFM

Even having an open draft RFC (probably for years) might allow us to establish and register the MYST parameter at least somewhat formally.

It is also worth noting that pandoc additionally provides for more parameters, that can specify which extensions/directives are loaded.

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[rowanc1]

Thanks @stevejpurves for brining this up! I worked with @choldgraf and @mmcky to get this done, and we have finished the registration process.

This is now super official!! 🎉🎉🎉🎉

text/markdown;variant=myst

We went with lowercase, similar to pandoc and quarto.

I will leave this up to document in the spec docs.