LibJS: Move GlobalObject to its own Object subclass

This is mostly for tidiness at the moment.

Libraries/LibJS/GlobalObject.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <LibJS/Object.h>
+
+namespace JS {
+
+class GlobalObject final : public Object {
+public:
+    explicit GlobalObject(Heap&);
+    virtual ~GlobalObject() override;
+
+private:
+    virtual const char* class_name() const override { return "GlobalObject"; }
+};
+
+}