LibJS: Clarify Object (base class) construction somewhat

Divide the Object constructor into three variants:

- The regular one (takes an Object& prototype)
- One for use by GlobalObject
- One for use by objects without a prototype (e.g ObjectPrototype)

Libraries/LibJS/Runtime/GlobalObject.cpp

@@ -64,7 +64,7 @@
 namespace JS {
 
 GlobalObject::GlobalObject()
-    : Object(nullptr)
+    : Object(GlobalObjectTag::Tag)
 {
 }