Separate non-hk specific stuff to a separate build

**Is your feature request related to a problem? Please describe.**
MagicUI is primarily unity stuff but is tightly coupled to HK, making it unreasonably difficult to port or reuse.

**Describe the solution you'd like**
Shift the architecture - this becomes MagicUI.Core, we create a new MagicUI.HollowKnight package that contains just the mod and setting interfaces

