LibWeb: Add basic support for DOM's NodeIterator and NodeFilter

This patch adds NodeIterator (created via Document.createNodeIterator())
which allows you to iterate through all the nodes in a subtree while
filtering with a provided NodeFilter callback along the way.

This first cut implements the full API, but does not yet handle nodes
being removed from the document while referenced by the iterator. That
will be done in a subsequent patch.

Userland/Libraries/LibWeb/Forward.h

@@ -110,6 +110,8 @@ class IDLEventListener;
 class LiveNodeList;
 class NamedNodeMap;
 class Node;
+class NodeFilter;
+class NodeIterator;
 class NodeList;
 class ParentNode;
 class Position;
@@ -469,6 +471,8 @@ class MessageEventWrapper;
 class MessagePortWrapper;
 class MouseEventWrapper;
 class NamedNodeMapWrapper;
+class NodeFilterWrapper;
+class NodeIteratorWrapper;
 class NodeListWrapper;
 class NodeWrapper;
 class PageTransitionEventWrapper;