Add mbstring to Github Actions

Author: casperbakker

.github/workflows/phpunit.yml

@@ -16,8 +16,9 @@ jobs:
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v1
-      with: 
+      with:
         php-version: ${{ matrix.php-versions }}
+        extensions: mbstring
 
     - name: Validate composer.json
       run: composer validate