Add default Dependabot config if one doesn't exist

On install we should add a default Dependabot config if there isn't an existing one.

It should be set up to create Pull Requests whenever a dependency should be updated according to its version constraint. This will ensure developers are notified and are given an easy system to update dependencies as soon as they are released.

In addition this will mean that teams can more easily rely on only using `composer install` locally to further ensure that their dependencies are in sync with each other.