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)

Added 1 commit:

• 27ab0671 - Add more tests + code cleanup

Added 2 commits:

• 2b0942a1 - Minor reformatting
• 574ffdd3 - Add documentation

Added 1 commit:

• 98a15535 - Add a /wip slash command

Added 158 commits:

• 98a15535...796bdf1d - 157 commits from branch gitlab-org:master
• 9d0b9635 - Add a /wip slash command

Marked the task CHANGELOG entry added as completed

Marked the task Documentation created/updated as completed

Marked the task Added for this feature/bug as completed

Marked the task Conform by the merge request performance guides as completed

Marked the task Conform by the style guides as completed

Marked the task Branch has no merge conflicts with master (if you do - rebase it please) as completed

Marked the task Squashed related commits together as completed

Marked the task All builds are passing as completed

Unmarked this merge request as a Work In Progress

Added 14 commits:

• 9d0b9635...483a28a4 - 13 commits from branch gitlab-org:master
• ffd9cc17 - Add a /wip slash command

Added 1 commit:

• ea8770d3 - Add a /wip slash command

@tbalthazar What happens if someone uses both /wip and /title in the same comment?