Add Rector to CI

This allows us to fail if Rector changes any code.

Also, this will fail Rector upgrades that need changes to the
configuration files (which is relevant for the Dependabot PRs).

.github/workflows/php.yml

@@ -63,3 +63,6 @@ jobs:
 
       - name: Unit Tests
         run: composer run-script ci:tests
+
+      - name: Rector
+        run: vendor/bin/rector --dry-run --no-progress-bar