Composer: allow the PHPCS plugin

[jrfnl]

The dealerdirect/phpcodesniffer-composer-installer Composer plugin is used to register external PHPCS standards with PHPCS.

As of Composer 2.2.0, Composer plugins need to be explicitly allowed to run. This adds the necessary configuration for that.

Refs:

• https://blog.packagist.com/composer-2-2/#more-secure-plugin-execution

[jrfnl]

Uh-oh... something weird going on with the tests - see: https://github.com/jrfnl/BrainMonkey/runs/4613262498?check_suite_focus=true

[jrfnl]

Rebased. Build is passing: https://github.com/jrfnl/BrainMonkey/actions/runs/1641062383