weekday static method
Implementation
static Validator<DateTime> weekday({String? error}) => (value, context) {
final DateTimeResource datetimeResource = context.resources.getOrCreate(
value.toString(),
() => DateTimeResource(value),
);
if (datetimeResource.isWeekday()) return (true, null);
return (false, error ?? context.errors.stringDateErrors.weekday());
};