Make TranslatorExtension extendable #354

Explain your intentions

I'd like to extend a custom TranslatorExtension from Latte's built-in one. I mainly need to encapsulate the translator function in a class, so that consumers can just do new CustomTranslator() without having to pass in a callback themselves. Like below. Unfortunately, this is currently impossible as TranslatorExtension is marked as final.

class CustomTranslator extends \Latte\TranslatorExtension
{
    public function __construct() {
      parent::__construct([$this, 'translate']);
    }

    public function translate($key, $data = null, $lang = null) {
        return app()->translate($key, $data, $lang);
    }
}

Make a strong case of the merits of this feature

I'm assuming there's no specific reason the class can't be extendable. If one wanted to do this, one would have to re-implement all the useful features inside it, like the _ tag and the |translate filter.