Add GitHub label action for merge conflicts

This helps to identify pull requests that have merge conflicts without
needing to check each PR individually or using some GraphQL magic.

This adds the "merge-conflict" label to conflicted PRs.

Signed-off-by: Tom Wieczorek <[email protected]>

.github/workflows/conflicts.yaml

@@ -0,0 +1,29 @@
+name: Check for merge conflicts
+
+on:
+  push:
+    branches:
+      - main
+      - release-*
+  pull_request_target:
+    types:
+      - opened
+      - reopened
+      - synchronize
+    branches:
+      - main
+      - release-*
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check for merge conflicts
+        uses: eps1lon/[email protected]
+        with:
+          repoToken: ${{ secrets.GITHUB_TOKEN }}
+          dirtyLabel: merge-conflict