GLTFLoader: Fix node parse (#25377)

* GLTFLoader: Fix node parse

This commit fixes a GLTFLoader node parse bug that
can cause an infinite loop in node parse - skin
parse - node parse.

The solution is adding ._loadNode() that parses a
single node without child nodes or skin to the parser.
.loadNode() and .loadSkin() call ._loadNode() to
get dependent nodes. ._loadNode() doesn't have
dependency with other nodes so infinite loop can
be avoided.

._loadNode() caches created nodes to avoid duplication
because it is called from two different methods,
.loadNode() and .loadSkin().

* GLTFLoader: Rename _loadNode to _loadNodeShallow

This commit renames _loadNode to _loadNodeShallow.

The behavior of the method is more clarified by
the new name so remove a commend that sounds like
duplicated with the method name (and also sounds
like a bit misleading).