add Mergify config for high priority PRs

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

Fixes: #10352

.github/mergify.yml

@@ -1,6 +1,12 @@
 # Note: We do not use the rebase strategy to merge PRs, because that
 # loses information needed by changelog-d to associate commits with PRs.
 
+priority_rules:
+  - name: "high priority :fire:"
+    conditions:
+      - label = "priority: high :fire:"
+    priority: high
+
 pull_request_rules:
 
   # implementing PR delay logic: apply a label after 2 days of inactivity