#13650 added wip search functionality and tests

What does this MR do?

Adds work in progress (WIP) filter to merge request (MR) search. Optional filters are "wip:yes" or "wip:no". If "yes" is selected, the search will return MR's with the prefix "WIP" or "[WIP]" or "WIP:". If "no" is selected, the search will return MR's without those prefixes.

@smcgivern @felipe_artur

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

Yes. Please double check that I was supposed to add "wip" to the scalar params array in the issueable_finder file.

Why was this MR needed?

Customers showed an interest in wanting to search by WIP, or filter them out.

Screenshots (if relevant)

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?

13650- Filter merge requests that are marked WIP

EE port https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6764