LibWeb: Share QualifiedName data between identical instances

Adopt the same pattern as AK::FlyString, reducing sizeof(QualifiedName)
to the size of a pointer.

This reduces the size of DOM::Element by 24 bytes.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -84,6 +84,7 @@ set(SOURCES
     DOM/ParentNode.cpp
     DOM/Position.cpp
     DOM/ProcessingInstruction.cpp
+    DOM/QualifiedName.cpp
     DOM/Range.cpp
     DOM/ShadowRoot.cpp
     DOM/StaticNodeList.cpp