Support :is() and :where() as aliases of :matches() #136

Problem

Neither engine supports :is(), the CSS Selectors 4 name for :matches(), nor :where(), which matches the same way but contributes no specificity (QueryPath doesn't use specificity, so it is an alias here too):

$xml = '<?xml version="1.0"?><ul><li id="a"/><li id="b"/></ul>';

qp($xml)->find('li:matches(#b)');  // b
qp($xml)->find('li:is(#b)');       // QueryPath\CSS\ParseException: Unknown Pseudo-Class: is
qp($xml)->find(':where(#b)');      // QueryPath\CSS\ParseException: Unknown Pseudo-Class: where

Proposal

Treat is and where as matches in CSS\DOMTraverser\PseudoClass, and add them to Util::SELECTOR_ARGUMENT_PSEUDO_CLASSES so a positional argument (:is(:first)) is applied to the set as it is for :matches(). Since #127 the legacy QueryPathEventHandler no longer backs any QueryPath method, so it can be left alone.

Raised in #123 and deferred from #127.