LibWeb: Add DOMParser

This allows you to invoke the HTML document parser and retrieve a
document as though it was loaded as a web page, minus any scripting
ability.

This does not currently support XML parsing.

This is used by YouTube (or more accurately, Web Components Polyfills)
to polyfill templates.

Userland/Libraries/LibWeb/Forward.h

@@ -59,6 +59,7 @@ enum class QuirksMode;
 namespace Web::HTML {
 class CanvasRenderingContext2D;
 class CloseEvent;
+class DOMParser;
 class EventHandler;
 class HTMLAnchorElement;
 class HTMLAreaElement;
@@ -211,6 +212,7 @@ class DocumentTypeWrapper;
 class DocumentWrapper;
 class DOMExceptionWrapper;
 class DOMImplementationWrapper;
+class DOMParserWrapper;
 class ElementWrapper;
 class EventListenerWrapper;
 class EventTargetWrapper;