Markdown preview image rendering (#21082)

Closes https://github.com/zed-industries/zed/issues/13246

Supersedes: https://github.com/zed-industries/zed/pull/16192

I couldn't push to the git fork this user was using, so here's the exact
same PR but with some style nits implemented.


Release Notes:

- Added image rendering to the Markdown preview

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Co-authored-by: dovakin0007 <dovakin0007@gmail.com>
Co-authored-by: dovakin0007 <73059450+dovakin0007@users.noreply.github.com>