WIP: Resolve "Add "gl-{number}" to reference issues and MR's"

What does this MR do?

This MR allows to use gli-<number> and glmr-<number> to refer to Issues and Merge Requests respectively

Are there points in the code the reviewer needs to double check?

I double checked if getting rid of Regexp.unescape in self.reference_pattern wouldn't be a problem but it's always a good idea to check this.

What's missing

I didn't really get around to looking for the popup that shows up under the text cursor when you start typing a ! or # so that would another thing to address.

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
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #24159 (moved)