Consider splitting SpaceTime library

[tomrijnbeek]

🆙 Improvement proposal

The space time concepts from this library hold significant value on their own, even outside the context of this library. We should consider whether we want to extract those into a separate library (even if said library is still in this repository).

Independently, but related, we should consider whether the space time values should be backed by System.Numerics types instead (see #146). That would open up the path towards making the space time library more universally useful. We can always support adapter libraries such as Bearded.SpaceTime.OpenTK and Bearded.SpaceTime.Unity which add extension methods to convert the underlying data to the respective types.

Context

Makes the library more universally useful.