[!!!TASK] Make all classes final

This library has a very large number of extension points via
interfaces and abstract classes. All other classes should not
be extended by users and shall theirfor be marked as final.

There are a few classes left that should also be marked as final
but those require some extra attention.

src/Markdown/DependencyInjection/MarkdownExtension.php

@@ -22,7 +22,7 @@
 
 use function dirname;
 
-class MarkdownExtension extends Extension implements PrependExtensionInterface, CompilerPassInterface
+final class MarkdownExtension extends Extension implements PrependExtensionInterface, CompilerPassInterface
 {
     /** @param mixed[] $configs */
     public function load(array $configs, ContainerBuilder $container): void