This adds an option to fill the ends of the generated mesh for
each spline combination with an N-gon. The resulting mesh is
manifold, so it can be used for operations like Boolean.

Theoretically, some operation like the fill curve could be used
to triangulate the caps with a combination of all profile splines.
However, performance is essential for this node since it is the
base of many workflows with curves, so that complexity doesn't
make sense here.