Use a dedicated node type to maintain the list of cached RVTs

While evaluating a stylesheet, result value trees (result tree fragments in the XSLT spec) are represented as xmlDocs and cached on the transform context in a linked list, using xmlDoc's prev and next pointers to maintain the list.

However, XPath evaluations can inadvertently traverse these links, which are an implementation detail and do not reflect the actual document structure. Using a dedicated node type avoids these unintended traversals.