Validates whether the input is a Polish VAT identification number (NIP).
v::nip()->assert('1645865777');
// Validation passes successfully
v::nip()->assert('1645865778');
// → "1645865778" must be a Polish VAT identification number
v::nip()->assert('1234567890');
// → "1234567890" must be a Polish VAT identification number
v::nip()->assert('164-586-57-77');
// → "164-586-57-77" must be a Polish VAT identification number
v::nip()->assert('164-58-65-777');
// → "164-58-65-777" must be a Polish VAT identification number