positive method
Requires the value to be positive (> 0).
Zero is not considered valid.
Example:
final validator = Checkit.num.positive().build();
print(validator.validate(5).isValid); // true
print(validator.validate(0).isValid); // false
print(validator.validate(-1).isValid); // false
Implementation
NumNode<T> positive({String? error}) {
_validators.add(NumValidator.positive(error: error));
return this;
}