Function validateVAT

validateVAT(vat: unknown): ValidationResult<string>
Validates a VAT registration number returning detailed ValidationResult.
Parameters
- vat: unknown
  VAT registration number.
Returns ValidationResult<string>
{ isValid: true, data } with the VAT number, or { isValid: false, error, errors, code }.
Example
```
validateVAT('123456789-7000').isValid; // true
```
- Defined in tax/validator.ts:94