Positional selectors in prevAll(), parents() and the *Until() methods count in document order, not jQuery's walk order #134

Problem

QueryPath counts a positional pseudo-class (:first, :last, :eq(), …) in a traversal's selector in document order. jQuery filters the walk's results in walk order, before sorting them, so the two disagree for the backwards traversals:

$xml = '<?xml version="1.0"?><r><a id="a"/><b id="b"/><c id="c"/><d id="d"/><e id="e"/></r>';

qp($xml, '#e')->prevAll(':first');   // QueryPath: a    jQuery: d (the nearest)
qp($xml, '#e')->prevAll(':last');    // QueryPath: d    jQuery: a
qp($xml, '#e')->prevAll(':eq(0)');   // QueryPath: a    jQuery: d

parents(':first'), parentsUntil() and prevUntil() differ the same way, on single and multi-element sets.

Cause

The positional filter runs inside DOMTraverser on its result set, which is in document order (#66), independently of the order the traversal method returns. #130 (#117) puts the returned results in jQuery's order and #131 (#118) pins the current positional results, both noting this as the remaining difference.

Proposal

Let callers that filter a walk pass the candidate order through to the positional filter (for example as part of DOMTraverser::filterNodes() from #128, which already preserves input order), and have the reverse-direction traversals filter in walk order. This is a behaviour change.

Results checked against jQuery 3.7.1 on jsdom.