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/CMakeLists.txt

@@ -72,6 +72,7 @@ set(SOURCES
     DOM/AbortController.cpp
     DOM/AbortSignal.cpp
     DOM/AbstractRange.cpp
+    DOM/AccessibilityTreeNode.cpp
     DOM/Attr.cpp
     DOM/Attr.idl
     DOM/ARIAMixin.cpp