Add a separate ruff GHA for inline hints

.github/workflows/ruff.yml

@@ -0,0 +1,22 @@
+name: Ruff
+
+on:
+  push:
+    branches:
+      - main
+      - 'stable/**'
+  pull_request:
+    branches: [main]
+
+jobs:
+  ruff:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - run: python -Im pip install --user ruff
+
+    - name: Run ruff
+      working-directory: ./src
+      run: ruff --output-format=github wagtail_headless_preview