add Mergify config for high priority PRs

[geekosaur]

This allows us to interrupt the merge queue for high priority PRs. It's using the "priority: high 🔥" label currently.

Fixes: #10352

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions). Mergify configuration is only read from master

[geekosaur]

This won't interfere with our usual use of the priority label, because we either commit those directly or set the "merge delay passed" label (usually to fix CI breakage). It's only if we set the label and a merge label that it will come into play. If anything, it will make the latter case work better — but since CI is almost always broken in that case, there probably won't be anything to interrupt.

[geekosaur]

Added an extra priority rule to automatically bump merges directly to release branches (i.e. not backports) on the grounds that we're probably in a release cycle in that case. High priority PRs still override these.

[geekosaur]

There's one more possible change to consider: if a PR has the "priority: high" label, automatically skip the cooldown period. That's a 3 line change to the code that adds the "merge delay passed" label, one of which is just a reindent. Note that you still have to set a merge label.

[ulysses4ever]

Yeah, auto-skip for the delay did for cross my mind. Would be a cool improvement, I think.

[geekosaur]

As it is spoken….