New API functions for retrieving ancestor and descentant content types

As requested in issue 90838, I made a nodefamily API function out of the proposed _nodeprofile_relation_descendants_dive() function.

It's slightly different than the original - it doesn't include the argument content type in its return array, in order to provide greater flexibility and behave in a similar way to nodefamily_get_{parent,child}_types(). For consistency with existing nodefamily functions, I renamed it nodefamily_get_descendant_types(). Also, I thought it appropriate to include a similar function for retrieving ancestors, called nodefamily_get_ancestor_types().

Let's see how you like it :D