Highlight extension

[tvanderstad]

Similar to #472 / #518, it would be great to be able to ==highlight== inline text.

Motivation
I'm working on a markdown editor for a note taking app that lets you edit the rendered markdown similar to Bear or Obsidian. For developers, highlights feel obscure (GitHub doesn't support them and I don't miss them) but for students it's a big deal. The "Seamless Markdown" section on Bear's website features a gif demonstrating markdown and highlighting is the first syntax they show. Obsidian also supports highlights. I'm just looking to match the competition.

I'm open to contributing if this is a change you'll accept. Is this a good first issue?

[kivikakk]

I'd love to have this feature in Comrak, please do!

(I note Pandoc supports a mark extension that does the same thing, but I'm inclined to think highlight would be a better name.)

[tvanderstad]

Okay! I'll aim to send a PR sometime in Q2. I'd like to ship a version of my app with comrak before I start adding features 😅

I found this PR from January adding admonition blocks - is that a good example to follow? Could you point me to an example for an inline?

I don't have a strong opinion on the name but if Pandoc calls it 'mark' and... should it render to html in a <mark> tag? If so it probably makes sense to call it 'mark' overall. It kind of keeps with the un-opinionated nature of the parser, like how it's called 'emphasis' instead of 'italic' and 'strong' instead of 'bold'.

[kivikakk]

There's some very similar WIP that I was unable to handle a few years ago in this PR: #302

<mark> does seem like the correct tag, given it's in HTML. Happy to call it mark overall; will let you make the choice!