Add Stat types

[greenstack]

Stats are common in turn-based games. Adding basic stats would be very useful.

Stats should be architected to support various numeric types and should eventually support some kind of pipeline pattern. It doesn't need to be exactly like this, but I'm willing to bet there's a lot to learn here. But that's out of the scope for this issue. For now, I just want there to be some kind of serializable container for these stat types.

[greenstack]

A feature allowing for generic math in C# is in the works. This has been the major roadblock on this issue, so my proposition is to put this on hold until .NET 6 is released and/or Unity has support for this feature (which who knows how long that will be). The Unity roadmap and this Visual Studio Magazine article may prove helpful.

[greenstack]

When we get INumeric support, I think having stats implement that interface may prove useful.