feat(v2): support comments for code highlighting

[elviswolcott]

Motivation

Per #2271, this adds the ability to highlight lines using highlight-next-line, highlight-start, and highlight-end comments.

Supported comment syntax:

• JSX - { /* comment */ }
• JS/C like single line - //comment
• JS/C like multi line - /* comment */
• Python - # comment
• HTML - <!-- comment -->

AFAIK most languages use at least one of these for comments.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

To test, use this markdown:
https://gist.github.com/elviswolcott/c4f15c6b0ec579e43a775cfdf9b25cc4

Related PRs

#2457 (docs)

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[elviswolcott]

I noticed some potential issues in the Gatsby implementation so I approached it a bit differently.

1. The line highlight comments are picked out of the syntax highlighted HTML. This leaves the highlighting dependent on knowing how Prism generates markup for certain code. It also doesn't lend itself to use in a component. By taking the comments from the raw code, it is easy to strip them out and pass in the highlight information without having any knowledge of what the markup will look like.
2. The regex expressions it uses are very loose. i.e. /* highlight-next-line --> seems to be considered a valid comment.

As I mentioned over in the issue, I've only added highlight-next-line, highlight-start, and highlight-end.

highlight-range would be trivial to add, but doesn't seem to really solve the issue of readability or maintainability the line ranges pose.

highlight-line would be simple to add (just a second pattern that doesn't check for a whole line), but it doesn't seem to have much value over highlight-next-line (especially as far as readability) so I've decided to keep it simple for now.

[markerikson]

OH THIS IS SO COOOOOOOL! :)

I'm hyped. Can't wait to use this!

[yangshun]

This is amazing. I can't wait to change all the code blocks to use this!