Add workflow to merge main into next (#79)

.github/workflows/main-into-next-pr.yml

@@ -0,0 +1,45 @@
+name: Create "Merge main into next" PR
+on:
+  push:
+    branches:
+      - main
+jobs:
+  main-into-next:
+    name: Create "Merge main into next" PR
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # Needed to also fetch next branch
+
+      - name: Setup Git user
+        run: |
+          git config user.name github-actions
+          git config user.email [email protected]
+
+      - name: Try automatic merge of main into next
+        id: automatic-merge-attempt
+        continue-on-error: true
+        run: |
+          git checkout next
+          git merge main
+          echo 'PR_TITLE=Merge main into next' >> $GITHUB_ENV
+          echo 'PR_BODY=This is an automated pull request to merge changes from `main` into `next`.' >> $GITHUB_ENV
+
+      - name: Merge with conflicts if automatic merge failed
+        if: steps.automatic-merge-attempt.outcome == 'failure' && steps.automatic-merge-attempt.conclusion == 'success' # https://docs.github.com/en/actions/learn-github-actions/contexts#steps-context
+        run: |
+          git add .
+          git commit -m "Merge main into next"
+          echo 'PR_TITLE=[Conflicts] Merge main into next' >> $GITHUB_ENV
+          echo 'PR_BODY=This is an automated pull request to merge changes from `main` into `next`. It has merge conflicts. To resolve conflicts, check out the branch `merge-main-into-next` locally, make any necessary changes to conflicting files, and commit and publish your changes.' >> $GITHUB_ENV
+
+      - name: Create pull request
+        uses: peter-evans/create-pull-request@v6
+        with:
+          token: ${{ secrets.VIVID_PLANET_BOT_TOKEN }}
+          title: ${{ env.PR_TITLE }}
+          body: ${{ env.PR_BODY }}
+          base: next
+          branch: merge-main-into-next