ci: prevent merge of stacked PRs

.github/workflows/stacked-prs.yaml

@@ -0,0 +1,18 @@
+name: Stacked PRs
+on:
+
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+  pull_request_target:
+    types: [opened, edited, synchronize, reopened]
+
+jobs:
+  prevent_merge_of_stacked_pr:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check that base is "main"
+        run: |
+          echo "${GITHUB_BASE_REF}"
+          if [[ "${GITHUB_BASE_REF}" != "main" ]]; then
+            exit 1;
+          fi