Support tables and LaTeX in markdown

[MattFisher]

Substack node types identified during the markdown-it rewrite (PR #61) that have no current Markdown support. Tables were called out explicitly in that PR as the intended first follow-up.

Tables

Standard GFM table syntax:

| Column A | Column B |
| -------- | -------- |
| Cell 1   | Cell 2   |

LaTeX / math blocks

$$...$$ fenced math syntax mapped to Substack's latex_block node:

{"type": "latex_block", "attrs": {"persistentExpression": "E=mc^2", "dirty": true}}

The mdit_py_plugins package ships a dollarmath plugin that handles parsing.

[ma2za]

Thanks. I can support this direction.

LaTeX looks scoped: mdit_py_plugins.dollarmath is already available, so after confirming the exact Substack latex_block JSON shape, this should be a renderer/node-builder change.

For tables, I would like to first capture a real Substack table from draft_body. Markdown-it can parse the table syntax, but we still need the Substack node schema before implementing it.