🛠️ Repo: Consolidate modules used for markdown processing

[boneskull]

See #3652 which may affect what happens here.

Currently we're using markdown-magic, markdownlint, and remark to handle linting and pre-processing various markdown files (including those used on mochajs.org).

I'm pretty sure everything here can be replaced by remark and its plugin ecosystem.

If so, let's see if we can do that. If the code ends up being less maintainable or otherwise more burdensome, forget it.

This does not have anything to do with the markdown reporter.