Enable testing merge queues @ GitHub Actions CI/CD (#93)

This allows org-hosted projects to start enabling merge queues in the
repository settings. With that, GitHub would trigger a separate event
against a merge commit derived from merging several pull requests with
the target branch.

.github/workflows/main.yml

@@ -1,6 +1,11 @@
 name: tests
 
-on: [push, pull_request]
+on:
+  merge_group:
+  push:
+    branches-ignore:
+    - gh-readonly-queue/**  # Temporary merge queue-related GH-made branches
+  pull_request:
 
 permissions:
   contents: read