Convert to .NET Core

[VariantXYZ]

I think we should still create a proper C# library for packet structures, but this can be handled later, and possibly in conjunction with PSO2Packet.

For now, as discussed yesterday, let's port the existing code in PolarisServer to .NET Core and work from there. We can obsolete the PolarisCore repo once that's done and continue work here.

[michael-fadely]

These days, this is easy as pie.