LibWeb: Move "element sibling" getters to NonDocumentTypeChildNode

Here's another CRTP mixin since that's the best we can do with C++.
This prepares exposing these via IDL on Element and CharacterData.

Libraries/LibWeb/DOM/Element.h

@@ -30,13 +30,17 @@
 #include <AK/String.h>
 #include <LibWeb/DOM/Attribute.h>
 #include <LibWeb/DOM/AttributeNames.h>
+#include <LibWeb/DOM/NonDocumentTypeChildNode.h>
 #include <LibWeb/DOM/ParentNode.h>
 #include <LibWeb/DOM/TagNames.h>
 #include <LibWeb/Layout/LayoutNode.h>
 
 namespace Web::DOM {
 
-class Element : public ParentNode {
+class Element
+    : public ParentNode
+    , public NonDocumentTypeChildNode<Element> {
+
 public:
     using WrapperType = Bindings::ElementWrapper;