Set up unity package branch with necessary metadata files

[grahamboree]

We'd like to ideally distribute dark config as a unity package for unity users to ease the initial integration process.

This involves creating a specific file structure, and adding few files like an asmdef and package manifest file as outlined here: https://docs.unity3d.com/Manual/CustomPackages.html

We should ideally do this in a way that doesn't make the main repo layout gross. I've seen this done as a branch in some other projects. We'd have to investigate the specifics of how those projects handle things without constantly needing to do branch merges or rebases.