Function tangent

tangent(
    point0: number,
    point1: number,
    point2: number,
    alpha: number,
    d0: number,
    d1: number,
): number
computes the tangent at point1, when alpha = 0: uniform 0.5: centripetal 1.0: chordal
Parameters
- point0: number
  Point 0
- point1: number
  Point 1
- point2: number
  Point 2
- alpha: number
  Alpha value
- d0: number
  Distance between point0 and point1
- d1: number
  Distance between point1 and point2
Returns number
Tangent value