Add frontend validation for VAT number syntax

[Tjitse-E]

This extension only exempts customers from VAT if their VAT Number is valid. What the extension does not do is validate the syntax. So it's possible for customers to enter the VAT Number with dots/spaces/etc.

Let's add a custom client side form validation to address this https://docs.hyva.io/checkout/hyva-checkout/devdocs/form-api/form-validations.html#client-side