Add a bot to label / unlabel PRs with merge conflicts

This is important information for reviewers, authors and people who are trying to merge PRs. As a result, I just added the `S-Merge-Conflicts` label.

Tracking and being alerted to this information is super tedious at scale though, and can be reliably automated.

We should figure out a way to do so! Like always, minimal permissions, minimal complexity, preference for dead-simple Bevy-owned tools written in Rust.

The issue names no files, tests, or entry points. Start by defining where a Bevy-owned Rust bot would run, how it can observe merge-conflict state, and the minimum permissions it needs. Done means pull requests are automatically labeled or unlabeled reliably as their conflict status changes.

Written by the indexing model from the issue text.