Thanks for suggesting this!

Although I've been hesitant to include new extensions in this library, I'm inclined to include this one because:

• The implementation is small, making the maintenance burden trivial
• Most (all?) parsers that do support highlights seem to use this syntax

The one unanswered question I have is whether mark is the correct return value for getXmlTagName() for this extension.

Yep, that works for me!

There are a few small changes I think we should make:

• Rename the extension itself to Highlight, but continue using Mark for the inline element type
• Fix the failing pipelines (code style, and copypaste errors in tests)
• Add this to the docs

I've got some free time now, so I'll give that a go :)

Thanks for contributing this! I'll include this in the next minor release (v2.8.0)