This new parameter allows developers to limit queried terms to terminal nodes -
ie, those without any descendants.
As part of the improvement, some internal logic in `get_terms()` has been
consolidated. Parameters that resolve to a NOT IN clause containing term IDs
('exclude', 'exclude_tree', and 'childless') are now parsed into a single
"exclusions" array before the SQL clause is generated.
Props theMikeD, horike.
Fixes#29839.
git-svn-id: https://develop.svn.wordpress.org/trunk@31275 602fd350-edb4-49c9-b593-d223f7449a82
c67e5da8c9