Beginnings of experimental MyST support

[irskep]

I had a grand vision of fully supporting both MyST and Djot syntax, since MyST has a much better role/directive feature.

Unfortunately, I found huge gaps in the JS implementation of myst-parser. I also don't like that it seems to be built on top of markdown-it, but then provides a data structure that is part of a completely different project (remark). And a few myst-parser dependencies pin old versions of markdown-it which trigger noisy deprecation warnings. Overall not ready for prime time.

That said, I'm not going to toss out all the work because it looks like adding remark support is probably the way forward. Then I can ditch the pandoc requirement for GFM and use remark extensions instead, and use mdast internally instead of the Djot AST, which has a lot of nice utilities available and is more syntactically rich.