CHORE(QA): add PHP Compatibility dependency

documentation:
- https://phpqa.io/projects/php-compatibility.html
- https://github.com/PHPCompatibility/PHPCompatibility

install
composer require --dev "phpcompatibility/php-compatibility"

Usage:
phpcs ./ -p --standard=PHPCompatibility --runtime-set testVersion 7.2-

Refs: #28