LibJS: Make Object.prototype.toString() fully spec compliant

- Fix evaluation order: IsArray(O) should always be called and before
  Get(O, @@toStringTag), previously it was the other way around and
  IsArray would only be called if @@toStringTag is not a string
- Add missing exception checks to both function calls
- Add missing builtin tag for arguments object

Also, while we're here:
- Update variable names to match spec
- Add spec step comments

Userland/Libraries/LibJS/Runtime/ArgumentsObject.cpp

@@ -18,6 +18,7 @@ ArgumentsObject::ArgumentsObject(GlobalObject& global_object, Environment& envir
 void ArgumentsObject::initialize(GlobalObject& global_object)
 {
     Base::initialize(global_object);
+    set_has_parameter_map();
     m_parameter_map = Object::create(global_object, nullptr);
 }