Function validate_add_to_variable_list 

pub fn validate_add_to_variable_list(
    key: &Token,
    _block: &Block,
    data: &Everything,
    sc: &mut ScopeContext,
    vd: Validator<'_>,
    _tooltipped: Tooltipped,
)

A specific validator for the three add_to_variable_list effects (global, local, and default). It is also used for the three remove_list_variable effects.