Create vertices from Spline

[DomCR]

Discussed in https://github.com/DomCR/ACadSharp/discussions/478

^{Originally posted by JorenPelfrene October 23, 2024} Hi, I find the PolyVertexes method very helpful for Arc and Ellipse entities. It would be great to also have this available for Spline (in the future). It's a handy feature when using ACadSharp together with the Clipper2 library for performing boolean operations on 2D geometry.

Thank you for your great work ;)