Validates if the input is countable.
Valid values:
validator.countable().validate([]);
validator.countable().validate(new Set());
validator.countable().validate(new Map());
Invalid values:
validator.countable().validate('foo');
validator.countable().validate(null);
validator.countable().validate(undefined);
validator.countable().validate(false);
validator.countable().validate(0);
validator.countable().validate({});