Moves prettier to dev dependency

What does this MR do?

This moves prettier into a dev dependency.

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

That prettier is moved to a dev dependency.

Why was this MR needed?

One point in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16217

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/41656

cc @timzallmann