add an option for 2d Delaunay for Mesh3d

For Mesh3d is there an easy option to tell plotly to form Delaunay triangulation using only the first two coordinates (x, y) ? I think one can form a triangulation oneself and pass it through but seems like delaunay can also do this and it's more IO to pass triangles on top of the data.

I think current implmentation only uses the three coords and that is why we have strange artifacts at the edges when plotting "surfaces".

https://plotly.com/python/3d-mesh/#mesh-tetrahedron

从 Mesh3d 的实现以及 issue 引用的 Delaunay 三角剖分路径开始。查看链接的 Mesh3d 文档示例，并确定当前三角剖分如何使用 x、y 和 z。完成的标准是：Mesh3d 提供仅使用 x 和 y 的 2D Delaunay 三角剖分选项，处理边缘伪影，并记录相关行为。