feat(ci): add backport PR action (#14766)

Trigger the backport action upon merge by marking a PR with the label `backport <branchname>` or after merge by applying the label and writing a comment containing `/backport`.

.github/workflows/backport.yml

@@ -0,0 +1,28 @@
+name: Backport
+on:
+  pull_request_target:
+    types: [closed, labeled]
+jobs:
+  backport:
+    name: Backport Pull Request
+    if: >
+      github.repository_owner == 'neovim' && (
+        github.event_name == 'pull_request' &&
+        github.event.pull_request.merged
+      ) || (
+        github.event_name == 'issue_comment' &&
+        github.event.issue.pull_request &&
+        contains(github.event.comment.body, '/backport')
+      )
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          # required to find all branches
+          fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha }}
+      - name: Create backport PRs
+        uses: zeebe-io/backport-action@v0.0.6
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          github_workspace: ${{ github.workspace }}