dom: register unprefixed id attributes from foreign content in HTML

[iliaal]

The HTML5 parser bridge only marked an id attribute as XML_ATTRIBUTE_ID when the attribute's lexbor namespace was LXB_NS_HTML, so ids on SVG and MathML elements never reached Dom\HTMLDocument::getElementById().

The condition the bridge wants is that the attribute is unprefixed, which lxml_attr->ns already records: the struct is memset to zero and ns is assigned only in the xmlns, xlink and xml branches above. Testing lxml_attr->ns == NULL covers every element namespace instead of an enumerated subset, and the test pins that a namespaced xml:id is still not registered.

[devnexen]

Suggested change

	if (local_name_length == 2 && local_name[0] == 'i' && local_name[1] == 'd' && lxml_attr->ns == NULL) {
	if (local_name_length == 2 && local_name[0] == 'i' && local_name[1] == 'd') {

[iliaal]

That also registers a genuinely namespaced id. lxml_attr->ns is non-NULL once a fragment is parsed into a foreign context:

$d = Dom\HTMLDocument::createEmpty();
$z = $d->createElementNS('http://www.w3.org/1999/xlink', 'z');
$d->appendChild($z);
$z->innerHTML = '<z id="q"></z>';
var_dump($d->getElementById('q'));

Without the check that returns the element, with it NULL. The test didn't cover the case, which is why the guard read as redundant; pinned in 3e45b39.