Render Markdown for error code explanations in rustc --explain

[camelid]

This would be really nice to have, but it might be a lot of work.

When I say "render Markdown", I mean to style it somewhat in the terminal: styling with bold or italic, showing links in a different color, syntax highlighting code snippets, etc. See gh, the GitHub CLI, for some examples of how we could do this.

[Mark-Simulacrum]

I don't think anything has changed since #63128; I am still inclined to say this is out of scope.

[camelid]

Ah, I tried searching for other issues but didn't find that one. Closing for now.