Include Primer styles from the primer-markdown npm module

[50Wliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Includes styles directly from the primer-markdown npm module so that we should stay up to date automagically. As far as I can tell this wasn't done originally because CSS files do not support mixins, however Less allows you to force CSS files to be interpreted as Less. Since all CSS files are Less files, this shouldn't be a problem.

I also deleted hr.png because it doesn't seem to be used anywhere? Maybe I'm wrong.

Alternate Designs

N/A

Benefits

Less code copy/pasting.

Possible Drawbacks

If the path to the compiled output changes, we'll need to update the path in markdown-preview-github.less as well.

Applicable Issues

/cc @thomasjo @simurai since you two were the ones that worked on the original PR.

[50Wliu]

Hmm. Now that I actually linked this package it seems like this won't work after all.