Add composer/installers to phpBB dependencies

This makes it easier for board admins to manage their extensions/styles/language packs with composer as it is not required to be added to every extension added to packagist.

See https://getcomposer.org/doc/articles/custom-installers.md, https://getcomposer.org/doc/faqs/how-do-i-install-a-package-to-a-custom-path-for-my-framework.md, https://github.com/composer/installers, and https://github.com/composer/installers/blob/master/src/Composer/Installers/PhpBBInstaller.php