CHORE(QA): add PHP_CodeSniffer (phpcs) dependency

documentation:
https://phpqa.io/projects/phpcs.html
https://github.com/squizlabs/PHP_CodeSniffer/wiki

# install
composer require --dev "squizlabs/php_codesniffer"

# Show rules and coding standards
vendor/bin/phpcs -i  #  list of installed coding standards
vendor/bin/phpcs -e  #  list of rules used by .phpcs.xml config

# Check files based on .phpcs.xml config
vendor/bin/phpcs

Refs: #27