Implement the solid angle measure (the plane angle) of a two-dimensional cone, using the dot-product (trigonometry) formula.

We do not assume nor compute the minimal description of the cone.
When the input contains more than two vectors as generators of the cone, we decompose the cone into simplicial cones. The helper function simplicial_subcones_decomposition will be re-used in higher dimension cases.

For example, the solid angle of the upper half plane (generated by three rays, hence not simplicial) solid_angle_2d([[1,0],[0,1],[-1,0]]) is 1/2.

Depends on #33656

CC: @yuan-zhou @mkoeppe

Component: geometry

Keywords: solid angle

Author: Allison Fitisone, Yuan Zhou

Branch/Commit: u/gh-allisonfitisone/implement_solid_angle_measure_for_two_dimensional_cones @ 7a7c253

Issue created by migration from https://trac.sagemath.org/ticket/33752