LibWeb: Add XMLDocument stub

Required by old versions of Pixi.js's XMLFormat bitmap font loader.
db824181dc/packages/text-bitmap/src/formats/XMLFormat.ts (L16)

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -155,6 +155,7 @@ set(SOURCES
     DOM/Text.cpp
     DOM/Text.idl
     DOM/TreeWalker.cpp
+    DOM/XMLDocument.cpp
     DOMParsing/InnerHTML.cpp
     DOMParsing/XMLSerializer.cpp
     Dump.cpp