Release Candidate for GrumPHP 2.x upgrade

* src/DrupalCheck.php

Updated for GrumPHP 2.x and include additional options from drupal-check

* src/ExtensionLoader.php
Updated for GrumPHP 2.x

* composer.json
Update composer.json to require GrumPHP 2.x.

* README.md
Updated list of options for drupal-check.

* Services.yaml
Added as part of GrumPHP 2.x compatibility.