fix: Add 'No VAT' VAT scheme constant

Author: olsgreen

src/Api/Enums/VatSchemes.php

@@ -6,5 +6,7 @@ class VatSchemes extends AbstractEnum
 {
     const MARGINAL = 'Marginal';
 
+    const NO_VAT = 'No VAT';
+
     const VAT_QUALIFYING = 'VAT Qualifying';
 }