LibWeb: Implement ParentNode.prepend

`convert_nodes_to_single_node` is inside its own file so ChildNode can
include and use it without having to include other headers such as
DOM/Node.h. This is to prevent circular includes.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -80,6 +80,7 @@ set(SOURCES
     DOM/LiveNodeList.cpp
     DOM/NamedNodeMap.cpp
     DOM/Node.cpp
+    DOM/NodeOperations.cpp
     DOM/ParentNode.cpp
     DOM/Position.cpp
     DOM/ProcessingInstruction.cpp