LibWeb: Implement fragment parsing and use it for Element.innerHTML

This patch implements most of the HTML fragment parsing algorithm and
ports Element::set_inner_html() to it. This was the last remaining user
of the old HTML parser. :^)

Libraries/LibWeb/DOM/Node.cpp

@@ -51,7 +51,7 @@
 namespace Web {
 
 Node::Node(Document& document, NodeType type)
-    : m_document(document)
+    : m_document(&document)
     , m_type(type)
 {
 }
@@ -212,4 +212,9 @@ RefPtr<Node> Node::insert_before(NonnullRefPtr<Node> node, RefPtr<Node> child, b
     return node;
 }
 
+void Node::set_document(Badge<Document>, Document& document)
+{
+    m_document = &document;
+}
+
 }