Validates if the input is a callable.
Valid values:
const obj: any = {
foo(): string {
return 'foo';
}
};
validator.callable().validate((): string => 'foo');
validator.callable().validate('eval');
validator.callable().validate('Array.isArray');
validator.callable().validate([obj, 'foo']);
Invalid values:
validator.callable().validate('');
validator.callable().validate('foo');
validator.callable().validate(null);
validator.callable().validate(undefined);