Notification when WIP Pull Request is ready for review

- Gitea version (or commit ref): 1.12.5+25-g3dc7f2839 (Codeberg.org)

## Description
Hey Gitea Team! :wave:

Just a quick feature request: It would be good if project maintainers could get an email notification when the "WIP" of a WIP Pull Request is removed, so that they know right away when the Pull Request is ready for review.

Thanks and keep up the good work! :)
