Adds remark-comment plugin to list

[leebyron]

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)
• If applicable, I’ve added docs and tests

Description of changes

https://github.com/leebyron/remark-comment

[wooorm]

Hey, I was already hoping that someone would create such a plugin!

• Would it be a good idea to perhaps give this a different name that alludes to MDX and HTML? E.g., remark-mdx-html-comment
• Should node.commentValue be changed to node.value? All nodes that have a literal string value, use the field value
• Instead of the literal 60 in the code, you could use codes.lessThan
• Should it instead expose the same AST as normal comments (probably not)? Or the same AST as {/*stuff*/}? (I like this?) — reason: that way it integrates with existing tools, such as those that turn on or off linting messages
• Probably good to add it here too: https://mdxjs.com/docs/extending-mdx/#list-of-plugins