enh(parser) add highlightElement, deprecate highlightBlock

[joshgoebel]

More work towards #2277.

Changes

• adds highlightElement(el) API and new plugin callbacks
• deprecates old highlightBlock(el) API and old callbacks
• both sets of plugin callbacks will be supported until v12

Supporting both sets of plugin callbacks should allow plugins to be written that easily work with both v10 and v11 (for a smooth transition for all).

Old API will continue to be supported until v12.

• Make sure API isn't double called if a plugin defines both for compatibility

Checklist

• Added markup tests, or they don't apply here because...
• Updated the changelog at CHANGES.md
• Added myself to AUTHORS.txt, under Contributors

[driesvints]

@joshgoebel could it be that this isn't reflected in the docs yet? highlightBlock isn't deprecated there yet and there's no docs on highlightElement. I'm seeing the below warnings but have no idea how to use the new highlightElement.

[joshgoebel]

The docs weren't building correctly. I fixed.

https://highlightjs.readthedocs.io/

Thanks. The API is the same for that method, only the name changed.

[driesvints]

@joshgoebel thanks for checking in on that! :)