Add CommonMark markdown engine

What does this MR do?

This MR adds new markdown engine called commonmarker next to the current one called redcarpet.
commonmaker is a wrapper for cmark-gfm (GitHub's fork of cmark).

The general idea is to replace redcarpet with commonmarker.
The migration is going to be smooth and step by step.

Please see #43011 (closed) for the discussion about migration.

Why was this MR needed?

This is the first step to migrate from the current markdown engine called Redcarpet to the new one called CommonMark.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#43011 (closed)