Replace commonmark.py with markdown-it-py

[bchopson]

Sounds like commonmark.py is going to get deprecated in favor of markdown-it-py.

Quoting a commenter on that issue:

I looked through all the markdown parsers for myst-parser, and well there is a reason markdown-it has 10 times the GitHub stars, and is used by VS Code etc; it's just a lot better, in terms of parsing logic, performance, extensibility, source mapping, etc