ci: create sync workflow

.github/workflows/sync.yml

@@ -0,0 +1,39 @@
+# This workflow tries to keep this repo in sync with upstream commits.
+# It creates a pull request merging upstream into this repo.
+# If the tests pass then the PR is automatically merged by GitHub.
+name: Sync upstream
+env:
+  UPSTREAM: pmret/papermario # If you forked dx, change this to star-haven/papermario-dx!
+  UPSTREAM_BRANCH: main
+  BASE_BRANCH: main
+on:
+  workflow_dispatch:
+  repository_dispatch:
+    types:
+      - webhook # 'push' event on upstream
+  schedule:
+    - cron: 0 0 * * *
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+          ref: ${{ env.BASE_BRANCH }}
+      - name: Fetch upstream
+        run: git fetch https://github.com/${{ env.UPSTREAM }}.git ${UPSTREAM_BRANCH}
+      - name: Merge upstream
+        run: |
+          git checkout ${BASE_BRANCH}
+          git merge --no-ff --log FETCH_HEAD -m "Merge upstream changes"
+      - name: Push changes
+        run: git push origin ${BASE_BRANCH}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Create Pull Request
+        run: |
+          gh pr create --title "Sync with ${{ env.UPSTREAM }}" --body "Automated PR to keep this repository in sync with upstream. Beep boop." --base ${BASE_BRANCH}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+