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/Document.h

@@ -145,6 +145,8 @@ public:
     bool in_quirks_mode() const { return m_quirks_mode; }
     void set_quirks_mode(bool mode) { m_quirks_mode = mode; }
 
+    void adopt_node(Node&);
+
 private:
     virtual RefPtr<LayoutNode> create_layout_node(const StyleProperties* parent_style) override;