Allow Hierarchy to delay computation of children

Allow Hierarchy to delay computation of children

This adds an option to allow callers to specify that computing
the children of the hierarchy should be delayed to when the
user calls for them, by utilizing the tree-widget :expander
property.
* lisp/emacs-lisp/hierarchy.el (hierarchy-add-tree)
(hierarchy-add-trees): Add parameter 'delay-children-p'.
* lisp/emacs-lisp/hierarchy.el
(hierarchy--create-delayed-tree-widget): Add function.
* lisp/emacs-lisp/hierarchy.el (hierarchy-convert-to-tree-widget):
Utilize ':expander' if delaying children.  (Bug#55900)

* test/lisp/emacs-lisp/hierarchy-tests.el: Add tests for
delayed-children functionality.