LibWeb: Add support for DOM's TreeWalker

This patch adds TreeWalker (created via Document.createTreeWalker())
which allows you to traverse a filtered view of the DOM in all
directions.

Userland/Libraries/LibWeb/DOM/TreeWalker.idl

@@ -0,0 +1,20 @@
+#import <DOM/Node.idl>
+#import <DOM/NodeFilter.idl>
+
+[Exposed=Window]
+interface TreeWalker {
+
+    [SameObject] readonly attribute Node root;
+    readonly attribute unsigned long whatToShow;
+    readonly attribute NodeFilter? filter;
+    attribute Node currentNode;
+
+    Node? parentNode();
+    Node? firstChild();
+    Node? lastChild();
+    Node? previousSibling();
+    Node? nextSibling();
+    Node? previousNode();
+    Node? nextNode();
+
+};