Add a GitHub action to block merging based on the labels

.github/workflows/do-not-merge.yml

@@ -0,0 +1,23 @@
+name: "Check labels"
+
+on:
+  pull_request:
+    branches: ["master"]
+    types:
+      - opened
+      - synchronize
+      - labeled
+      - unlabeled
+  merge_group:
+    branches: ["master"]
+    types: [checks_requested]
+
+jobs:
+  fail-by-label:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fail if PR is labeled as "do not merge"
+        if: contains(github.event.pull_request.labels.*.name, 'do not merge')
+        run: |
+          echo "This PR is labeled as do not merge!"
+          exit 1