Composer/CaptainHook: Install the `Cptn` when user `Composer`.

[mjansenDatabay]

This commit adds the captainhook/plugin-composer to our development dependencies. With this we can ensure that every developer will install it and use our shared hooks. We tried to achieve this manually with https://github.com/ILIAS-eLearning/ILIAS/pull/3724 and https://github.com/ILIAS-eLearning/ILIAS/pull/3839, but relying on this plugin is the superior approach.