GitHub CSS is out of date

MacDown bundles `GitHub.css` and `GitHub2.css` for use as rendering options. Both were last updated 4 years ago and seem to be jarringly different from the current rendering on GitHub.

I see `Tools/GitHub-style-generator` exists in this repo and appears to be designed to use the rather official looking `primer-markdown` (now superseded by `primer-css`) to derive the current GitHub markdown css. However, it does not seem to have been used to update what gets bundled with the app lately.

In the meantime, I used [sindresorhus/github-markdown-css](https://github.com/sindresorhus/github-markdown-css) to manually produce a suitable [CSS file](https://gist.github.com/hraftery/e8afa1a55848b2a626295007ea78f59b) that is working well for me. I've documented the method I used [here](https://twitter.com/HeathRaftery/status/1517372992757518336?s=20&t=rl9_xKBnki0e-D0AlhF_Pg). One clarification - I got the `padding` properties from `GitHub2.css`.

While this file solves the issue for me for now (and may help others), it probably does not belong in the repo because it's not autogenerated. I feel like either `Tools/GitHub-style-generator` or [sindresorhus/generate-github-markdown-css](https://github.com/sindresorhus/generate-github-markdown-css) would be a better source.

Start by reviewing Tools/GitHub-style-generator and the bundled GitHub.css and GitHub2.css, then compare their output with the current GitHub Markdown styling or the proposed sindresorhus generator. Done means the bundled rendering styles are updated from a reproducible source and the manual gist is not required.

Written by the indexing model from the issue text.