ci: add backport action (#253)

.github/workflows/backport.yml

@@ -0,0 +1,33 @@
+# this is based on https://github.com/NixOS/nixpkgs/blob/c81ecdf95b3a0f73ded448f14416cd66beeb5e1a/.github/workflows/backport.yml
+name: Backport
+on:
+  pull_request_target:
+    types: [closed, labeled]
+
+# WARNING:
+# When extending this action, be aware that $GITHUB_TOKEN allows write access to
+# the GitHub repository. This means that it should not evaluate user input in a
+# way that allows code injection.
+
+jobs:
+  backport:
+    name: Backport Pull Request
+    if: github.repository_owner == 'catppuccin' && github.event.pull_request.merged == true && (github.event_name != 'labeled' || startsWith('backport', github.event.label.name))
+
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: write
+      pull-requests: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+
+      - name: Create backport PRs
+        uses: korthout/backport-action@v2
+        with:
+          pull_description: |-
+            Bot-based backport to `${target_branch}`, triggered by a label in #${pull_number}.