Add unity package manager support

[yanicksenn]

I was wondering whether there is any reason to not support unity's package manager?

This would require at least a package.json in the root to make it work:
https://docs.unity3d.com/6000.0/Documentation/Manual/cus-layout.html

We could theoretically also change the folder structure / name to match the common layout (however, this is not necessarily needed and is just a nice-to-have).

I would also propose to the PR if you're supportive of the idea.