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/SymbolConstructor.cpp

@@ -11,7 +11,7 @@
 namespace JS {
 
 SymbolConstructor::SymbolConstructor(GlobalObject& global_object)
-    : NativeFunction(vm().names.Symbol, *global_object.function_prototype())
+    : NativeFunction(vm().names.Symbol.as_string(), *global_object.function_prototype())
 {
 }