Add Reviewers Quick Action

Adding GitLab Quick Action for Merge Requests

https://docs.gitlab.com/ee/user/project/quick_actions.html#gitlab-quick-actions

Here's the assignee command for reference

For now, we would just want to cover the MR section.

Proposal

We could proceed with /assign_reviewer, /reassign_reviewer, /unassign_reviewer in a first iteration, adding /reviewer and /remove_reviewer later as aliases if needed.