Add actions/labeler (#26962)

Implements https://github.com/GiteaBot/gitea-backporter/issues/93 using
[`actions/labeler`](https://github.com/actions/labeler). Very basic
configuration, can be extended later.

.github/workflows/pull-labeler.yml

@@ -0,0 +1,21 @@
+name: labeler
+
+on:
+  pull_request_target:
+    types: [opened, synchronize, reopened]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - uses: actions/labeler@v4
+        with:
+          dot: true
+          sync-labels: true