Added package.json for Package Manager support.

[vncastanheira]

This file allows SabreCSG to be added as a Package in Unity projects.

[Henry00IS]

This also requires an asmdef file, which then breaks all of the reflection code. It won't find additional compound brushes and volumes in your project (nor the included ones). There's also a lot of hardcoded logic that checks the Assets/SabreCSG directory. It's not quite as easy as just adding a json file.