LibWeb: Add Function To Build Accessiblity Tree

With this patch, the accessibility tree can be build from the root
node of a document. This can then be serialzed and sent to (soon
to come) consumers.

Userland/Libraries/LibWeb/Forward.h

@@ -136,6 +136,7 @@ namespace Web::DOM {
 class AbstractRange;
 class AbortController;
 class AbortSignal;
+class AccessibilityTreeNode;
 class Attr;
 class CDATASection;
 class CharacterData;