MethodPrototypeSymbol::serialize: write subroutine, if it exists

[tdp2110]

As in 47010bdd (NewClassExpression::serializeTo: write isSuperClass property, Wed Apr 24 15:17:01 2024 -0400), the subroutine member of a MethodPrototypeSymbol is a crucial property for understanding the prototype. Write it in the serializer (when it exists), so that consumers, such as slang-explorer, can see it.