fix(BY): allow digits in the Belarus bank identifier (4!c)

[spokodev]

Problem

The Belarus BBAN structure is 4!c4!n16!c per the ISO 13616 / SWIFT IBAN Registry, where the leading 4-character bank identifier is 4!c — alphanumeric (digits or uppercase letters). The current spec encodes it as [A-Z]{4} (letters only):

BY: { chars: 28, bban_regexp: '^[A-Z]{4}[0-9]{4}[A-Z0-9]{16}$', ... }

As a result, an otherwise-valid Belarus IBAN whose bank code contains a digit is rejected:

const { isValidIBAN } = require('ibantools');

isValidIBAN('BY13NBRB3600900000002Z00AB00');   // true  (bank code = letters)
isValidIBAN('BY26AKB10100000002966000000A');   // false (bank code "AKB1" — BUG)

BY26AKB10100000002966000000A is well-formed: 28 characters, BBAN 4!c4!n16!c, and its mod-97 check digits are valid (remainder 1). The only reason it fails is the bank field rejecting the digit 1.

Fix

Change the bank field from [A-Z]{4} to [A-Z0-9]{4}. The neighbouring branch field [0-9]{4} (4!n) and account field [A-Z0-9]{16} (16!c) are already correct, so this aligns the bank field with the same registry, consistent with prior BBAN-format corrections in this project.

References

• SWIFT IBAN Registry (ISO 13616): Belarus BBAN 4!c4!n16!c, length 28.

Test

Added an assertion that a Belarus IBAN with a digit in the bank code validates. It fails before the change and passes after; the full suite is green.