Make internal classes final

.php-cs-fixer.dist.php

@@ -1,7 +1,7 @@
 <?php
 /*
  * This document has been generated with
- * https://mlocati.github.io/php-cs-fixer-configurator/#version:3.12.0|configurator
+ * https://mlocati.github.io/php-cs-fixer-configurator/#version:3.16.0|configurator
  * you can change this configuration by importing this file.
  */
 $config = new PhpCsFixer\Config();
@@ -42,6 +42,7 @@
 		'dir_constant' => true,
 		'elseif' => true,
 		'encoding' => true,
+		'final_internal_class' => true,
 		'full_opening_tag' => true,
 		'fully_qualified_strict_types' => true,
 		'function_declaration' => ['trailing_comma_single_line'=>true],