Added $loader variable typehint for easier manipulation in IDEs.

Not all project dependencies are managed by Composer. Common solutions for (company / private) libraries and bundles are git submodules or even symlinked directories.

This PR adds typehint for `$loader` variable inside `app/autoload.php` so people won't need to search for information on what class it is and what interface it exposes when trying to add their code to autoloader.