chore: add action to auto add prs to project board

.github/workflows/prs.yml

@@ -0,0 +1,16 @@
+name: Add pull requests to Tiptap project
+
+on:
+  pull_request:
+    types:
+      - opened
+
+jobs:
+  add-to-project:
+    name: Add pull request to project
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/add-to-project@main
+        with:
+          project-url: ${{ secrets.ADD_TO_PROJECT_PR_URL }}
+          github-token: ${{ secrets.ADD_TO_PROJECT_PAT }}