Add workflow to comment on a PR when it was dequeued

Author: mbg

.github/workflows/pr-dequeued.yml

@@ -0,0 +1,26 @@
+name: PR dequeued
+
+on:
+  pull_request:
+    types:
+      - dequeued
+
+jobs:
+  notify:
+    name: Notify
+    runs-on: ubuntu-slim
+
+    permissions:
+      pull-requests: write
+
+    steps:
+      - name: Notify that the PR was removed from the merge queue
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          REASON: ${{ github.event.reason }}
+          SENDER: ${{ github.event.sender }}
+        run: |
+          echo "@$SENDER The pull request was removed from the merge queue (`$REASON`)." \
+            "Please resolve any issues if required and enqueue the PR again." |
+            gh pr comment --body-file - --repo github/codeql-action "$PR_NUMBER"