workflows: add spell checker for PR changes

.github/workflows/extra-pr-checks.yml

@@ -0,0 +1,31 @@
+name: Test Pull Request
+
+on: 
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  check-spelling:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Install dependencies
+        run: python -m pip install codespell
+      - name: Checkout source
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 2  # Fetch the latest state + the PR changes
+      - name: Get modified files
+        id: changed-files
+        run: |
+          echo "changed_files=$(git diff --name-only HEAD^1 HEAD | xargs)" >> $GITHUB_OUTPUT
+      - name: Check spelling of added content
+        run: |
+          # Process additions to each file, and count files that have new lines with errors
+          set +e  # Don't fail early
+          errors=0
+          for file in ${{ steps.changed-files.outputs.changed_files }}; do
+            git diff HEAD^1 HEAD $file | sed -n 's/^\+//p' > addition
+            echo -e "\nChecking $file"
+            codespell addition || ((errors++))
+          done
+          exit $errors