Publish `SMAPI.Toolkit` and `SMAPI.Toolkit.CoreInterfaces` to NuGet

[erri120]

The SMAPI.Toolkit and SMAPI.Toolkit.CoreInterfaces projects contain utilities useful for external tools. However, these projects are not available as NuGet packages. Currently, the only viable option of using these projects is via git submodules.

[ImKringle]

+1