Follow up to https://github.com/zed-industries/zed/pull/54374 as that
removed the custom heading styles that were mostly tailored to the agent
panel. It makes sense for those values not to be the ones used for
general markdown rendering, but I think it's just too big for the agent
panel. So I'm sort of hijacking the `MarkdownFont::Agent` enum here so
that we can set it conditionally to just the agent context.

Release Notes:

- N/A