CI: add backport workflow

Refs: https://codeberg.org/forgejo/docs/issues/18

.forgejo/workflows/backport.yml

@@ -0,0 +1,29 @@
+name: backport
+
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    runs-on: docker
+    container:
+      image: node:20-bookworm
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
+    steps:
+      - name: send backport if the label "backport {branch}" is present
+        uses: https://github.com/tibdex/backport@v2
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}