Refactored Curve.js to individual classes (#3378)

Split /src/extras/core/Curve.js to
/src/extra/curves/*

LineCurve
QuadraticBezierCurve
CubicBezierCurve
SplineCurve
EllipseCurve
ArcCurve
LineCurve3
QuadraticBezierCurve3
CubicBezierCurve3
SplineCurve3
ClosedSplineCurve3