LibJS: Accept FlyStrings in the NativeFunction constructors

This makes the implicit run-time assertion in PropertyName::to_string()
into an explicit compile-time requirement, removes a wasteful FlyString
-> PropertyName -> FlyString construction from NativeFunction::create()
and allows setting the function name to a null string for anonymous
native functions.

Userland/Libraries/LibJS/Runtime/ObjectConstructor.cpp

@@ -18,7 +18,7 @@
 namespace JS {
 
 ObjectConstructor::ObjectConstructor(GlobalObject& global_object)
-    : NativeFunction(vm().names.Object, *global_object.function_prototype())
+    : NativeFunction(vm().names.Object.as_string(), *global_object.function_prototype())
 {
 }