NuGet Package Support

We are writing T4 files to add additional support across a wide number of struct types Mathematics provides (ex. float2, double2, half2, float3, etc.). Because we need direct control of the csproj for code generation, we have to do this in a separate library rather than inside of Unity. It would be useful if there were capabilities to reference the Unity.Mathematics library from outside of a Unity project, with NuGet for example. It seems that for our current use case, Unity's Package Manager would not be able to resolve this issue. Currently we are having to generate builds and copy the Unity.Mathematics.dll to a lib folder to reference.

The issue describes Unity.Mathematics.dll generation and copying it into a lib folder for external projects. Start by reviewing the repository's current build and package configuration, then determine how an external NuGet consumer would reference the library. Done means the library can be consumed outside Unity through NuGet instead of manual DLL copying.