Map DOMDocument::class to Gt\Dom\HTMLDocument::class (#449)

* Map DOMDocument::class to Gt\Dom\HTMLDocument::class

Fixes `DOMDocument::registerNodeClass(): Argument #2 ($extendedClass) must not be an abstract class` from change in PHP 8.1.26

* Map DOMDocument::class to static::class

Ensures DOMDocument is always mapped to the class that extends abstract Gt\Dom\Document

src/Document.php

@@ -36,7 +36,7 @@ abstract class Document extends DOMDocument implements Stringable, StreamInterfa
 	use RegisteredNodeClass;
 
 	const NODE_CLASS_LOOKUP = [
-		DOMDocument::class => Document::class,
+		DOMDocument::class => self::class,
 		DOMAttr::class => Attr::class,
 		DOMCdataSection::class => CdataSection::class,
 		DOMCharacterData::class => CharacterData::class,
@@ -357,6 +357,9 @@ public function writeln($line):int {
 
 	private function registerNodeClasses():void {
 		foreach(self::NODE_CLASS_LOOKUP as $nativeClass => $gtClass) {
+			if ($gtClass === self::class) {
+				$gtClass = static::class;
+			}
 			$this->registerNodeClass($nativeClass, $gtClass);
 		}
 	}