Add PHP extension mbstring

Author: Kevinrob

.github/workflows/tests.yml

@@ -15,6 +15,7 @@ jobs:
       uses: shivammathur/setup-php@1.7.0
       with:
         php-version: ${{ matrix.php-versions }}
+        extensions: mbstring
 
     - name: Validate composer.json and composer.lock
       run: composer validate