Add PHP Code Sniffer to project for CI testing

[geerlingguy]

I want to have a PHP standards check in the CI workflow on GitHub (and available locally) to ensure my site's codebase conforms to Drupal's coding standards. See: https://www.drupal.org/docs/8/modules/code-review-module/installing-coder-sniffer

It seems that the best way to do this is:

1. Install Coder: composer require --dev drupal/coder dealerdirect/phpcodesniffer-composer-installer
2. Test that it's working: ./vendor/bin/phpcs -i