Update MDX, support version 1 and 2, as well as the multiple ASTs generated

[ChristianMurphy]

mdx version 1 and version 2 have different syntax trees, this PR allows both versions' syntax trees to be previewed.

In addition, mdx internally uses multiple syntax trees.

Version 1 has:

• a markdown AST (mdast/remark)
• an HTML AST (hast/rehype)

Version 2 has:

• a markdown AST (mdast/remark)
• an HTML AST (hast/rehype)
• a JavaScript AST (esast/recma)

This updates AST Explorer to have a preview for all version 1 and version 2 ASTs.
As well as updates transformers to allow for transforms on both version 1 and version 2.

This also adds support for syntax extensions, supported in MDX version 2; including GFM, Math, Directives, and more.

/cc @silvenon, @remcohaszing, @wooorm

---

Screenshots

mdx 2 preview with mdx 2 transformer

parser list for mdx

mdx 2 settings/plugins

[remcohaszing]

Thanks! I really love astexplorer, but I miss MDX2 support

[wooorm]

Any reason the old AST is still around? Why not drop it?

[remcohaszing]

I also wouldn't mind dropping the old AST

[ChristianMurphy]

Any reason the old AST is still around? Why not drop it?

A couple reasons.

1. there are still plugins out there for mdx 1, which could benefit from having a tool to help inspect ASTs
2. ASTExplorer has some precedence with keeping multiple major versions of parsers. For example:
   • three major versions of the babylon parser
   • four major versions of babel eslint
   • five major versions of eslint

I'd also be fine with dropping it, but defaulted to keeping it.

[wooorm]

Re 2: Old versions seems to be around to support old snippet links, they seem to be hidden: fad47c2

Re 1: MDX 0 and 1 ASTs, IMO, are more the “absence” of an AST. It’s mdast, but replaces a html node with a jsx node, with a slightly different regex. And it adds import/export nodes, which are similarly strings, with different regexes.
I don’t think it’s very interesting to look at this AST. Now that there are nodes for things in v2, I do think it gets interesting, and we could start versioning to keep snippet links alive?

[remcohaszing]

I suppose MDX 3 should be added by now too.