actions: automatically add a "needs rebase" label and comment as relevant

.github/workflows/label-merge-conflicts.yml

@@ -0,0 +1,21 @@
+name: "Label Merge Conflicts"
+on:
+  # So that PRs touching the same files as the push are updated
+  push:
+  # So that the `dirtyLabel` is removed if conflicts are resolve
+  # We recommend `pull_request_target` so that github secrets are available.
+  # In `pull_request` we wouldn't be able to change labels of fork PRs
+  pull_request_target:
+    types: [synchronize]
+
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - name: check if prs are dirty
+        uses: eps1lon/actions-label-merge-conflict@releases/2.x
+        with:
+          dirtyLabel: "needs rebase"
+          repoToken: "${{ secrets.GITHUB_TOKEN }}"
+          commentOnDirty: "This pull request has conflicts, please rebase."
+          #commentOnClean: "Conflicts have been resolved. A maintainer will review the pull request shortly."