LibWeb: Implement Structured{De}SerializeWithTransfer for transferables

This first cut at these APIs only works for platform objects that
implement the Transferable interface from Bindings.

Userland/Libraries/LibWeb/Bindings/Intrinsics.h

@@ -57,6 +57,8 @@ public:
         return *m_constructors.find(class_name)->value;
     }
 
+    bool is_exposed(StringView name) const;
+
 private:
     virtual void visit_edges(JS::Cell::Visitor&) override;