Add a ThreePointCubic spline that combines two cubic curves (#80836)

This adds a "three point" cubic curve that combines two cubic curves into one curve, where the control points are all in the same coordinate system. It's called "three point" because it incorporates a middle point that the curve runs through, and that point is in addition to the existing implicit (0,0) and (1,1) points that a Curve already has.

I also added an example of this curve, which is just a more emphasized version of the easeInOutCubic.