Enable merge queues

[masklinn]

From time to time I have two PRs I want to merge (generally because I extracted one from the other) and in that case github's auto-merge is just stupid: with "require branches to be up to date before merging" once the first PR has passed its checks and been merged the second will just wait forever with no notification sent to anyone

https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#configuring-continuous-integration-ci-workflows-for-merge-queues