LibWeb: Implement HTML fragment serialisation and use it in innerHTML

The previous implementation was about a half implementation and was
tied to Element::innerHTML. This separates it and puts it into
HTMLDocumentParser, as this is in the parsing section of the spec.

This provides a near finished HTML fragment serialisation algorithm,
bar namespaces in attributes and the `is` value.

Userland/Libraries/LibWeb/DOM/Node.h

@@ -183,6 +183,8 @@ public:
     i32 id() const { return m_id; }
     static Node* from_id(i32 node_id);
 
+    String serialize_fragment() const;
+
     void replace_all(RefPtr<Node>);
     void string_replace_all(String const&);