Add type 'project' to composer.json

That is what is suggested by https://getcomposer.org/doc/04-schema.md#type

> project: This denotes a project rather than a library. For example application shells like the Symfony standard edition, CMSs like the SilverStripe installer or full fledged applications distributed as packages. This can for example be used by IDEs to provide listings of projects to initialize when creating a new workspace.