Add VAT number examples

[PierrickP]

Can be used on placeholder or error message.

Usage:

import { getVATExample, austria } from 'jsvat';

getVATExample(austria) // ATU12345678

Based on Stripe examples https://stripe.com/docs/billing/customer/tax-ids

These numbers are not valid, only the format

[mathieumagalhaes]

Since this repo seemed so abandoned I've created a fork on: https://github.com/mathieumagalhaes/jsvat-next

I was wondering if the community wants this change in this library, or if this is out of scope.