Support GitHub alerts

[nzakas]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

GitHub added support for alerts at the end of 2023. This package doesn't currently parse this syntax.

Solution

Update mdast-util-gfm to support alerts.

Alternatives

Another approach would be to create a separate plugin just for alerts so anyone could mix it into their setup. However, it does seem like most people expect "GFM" to include alerts at this point.

[remcohaszing]

GitHub alerts are not a part of GFM. It’s an HTML transform. See rehypejs/rehype-github#3 and https://github.com/remarkjs/remark-gfm/issues?q=is:issue+alert.