Add grumphp linting to PR checker.

.github/workflows/php.yml

@@ -33,6 +33,9 @@ jobs:
     - name: Install dependencies
       run: composer install --prefer-dist --no-progress
 
+    - name: Linting w/ Grump
+      run: git ls-files | php ./vendor/bin/grumphp run --verbose
+
     # Add a test script to composer.json, for instance: "test": "vendor/bin/phpunit"
     # Docs: https://getcomposer.org/doc/articles/scripts.md