Add built-in cone mesh type.

Also delete `prism` type since it is subsumed by cone.

PiperOrigin-RevId: 790992343
Change-Id: I7a9c6f2a4377489173acd64b23086c748d2cee8e

src/user/user_objects.h

@@ -1053,7 +1053,7 @@ class mjCMesh: public mjCMesh_, private mjsMesh {
   // make a mesh of a predefined shape
   void MakeWedge(int resolution[2], double fov[2], double gamma);
   void MakeRect(int resolution[2]);
-  void MakePrism(int nedge);
+  void MakeCone(int nedge, double radius);
 
   // accessors
   const mjsPlugin& Plugin() const { return plugin; }