Support adding and removing point(s) from a spline

[ecurtiss]

This would be a substantial performance improvement over the current behavior, which is to reconstruct the entire spline when its control points change. Care would need to be taken if the user had already called PrecomputeUnitSpeedData() or PrecomputeRMFs().

One use case for this is adding and removing points in a curve editor plugin such as https://devforum.roblox.com/t/2983354.