prevAll(), closest() and other traversals don't return results in jQuery's order #117

Follow-up from #72. Assumes #72 is merged.

Problem

jQuery returns the results of its reverse-direction traversals in reverse document order, de-duplicated (.parents(), .parentsUntil(), .prevAll(), .prevUntil()), and runs uniqueSort on the others. #72 brought parents() and parentsUntil() in line, but the rest still return results grouped by starting element, in walk order:

$xml = '<r><a/><b/><c/><d/></r>';

qp($xml, 'c, d')->prevAll();
// QueryPath: b, a, c, b, a  → after de-dup: b, a, c   (grouped per start node)
// jQuery:    c, b, a                                  (reverse document order)

This matters beyond presentation: since #70, positional selectors (:first, :eq(n), …) count within the set, so prevAll(':first') picks a different element depending on the method's ordering. The CHANGELOG's "position within the set" guarantee currently holds only for some methods.

Affected

prevAll(), prevUntil(), closest(), parent(), siblings(), next()/prev() on multi-element sets. Each needs checking against jQuery 3.x.

Proposal

Decide the order per method from jQuery's source, and apply it in one place (ideally the shared traversal helper from the consolidation issue) rather than per method. This is a behaviour change and should be marked as one in the CHANGELOG.