Support for MyST syntax

[asmeurer]

It would be useful to have a mode to support MyST syntax. MyST is an extension of CommonMark that adds support for various RST-style markup for use with Sphinx. It is gaining popularity as a replacement for RST with the Sphinx document system, especially in the Python ecosystem.

You can see the full list of MyST syntax here https://myst-parser.readthedocs.io/en/latest/syntax/syntax.html. In particular, it would be nice if it didn't consider ```{directive} to be a code block, as typical directives just use Markdown syntax inside of them. Right now, it renders every directive block as a code block, which removes all syntax highlighting and other features inside of it, and even disables flyspell spell checking.

Syntax highlighting of things like {ref}`target` and (target)= would also be useful.

MyST syntax is a superset of CommonMark, so I would imagine that just like gfm-mode it wouldn't be too hard to add a custom mode for it with at least basic support.