Add a /wip slash command to toggle the 'WIP' prefix in the MR title

What does this MR do?

It adds a /wip slash command that toggles the WIP: prefix in the MR title.

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

N/A

Why was this MR needed?

As explained in #21225 (closed), it prevents you from having to click through the entire edit form just to mark an MR as WIP.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21225 (closed)