Phar abstraction

This PR introduces a layer between the phar abstraction from an external library and our own business logic, encapsulating everything related to adding things like files or directories to the target phar. It aids in expressing more clearly our own business domain, namely creating a phar file, which in turn uses the external `Herrera\Box\Box` as its technical implementation. Additionally it enables us to prevent leaking implementation details into our business logic because the API of aforementioned class exposes that it works with PHP's own `Phar` class.