Prevent PRs with the X-Blocked label from being merged (#130)

Adapted from matrix-js-sdk (https://github.com/matrix-org/matrix-js-sdk/blob/develop/.github/workflows/pull_request.yaml)

Author: robintown

.github/workflows/blocked.yaml

@@ -0,0 +1,17 @@
+name: Prevent blocked
+on:
+    pull_request_target:
+        types: [opened, labeled, unlabeled]
+jobs:
+    prevent-blocked:
+        name: Prevent blocked
+        runs-on: ubuntu-latest
+        permissions:
+            pull-requests: read
+        steps:
+            - name: Add notice
+              uses: actions/github-script@v7
+              if: contains(github.event.pull_request.labels.*.name, 'X-Blocked')
+              with:
+                  script: |
+                      core.setFailed("PR has been labeled with X-Blocked; it cannot be merged.");