feat(validation): Implement EN16931 compliance validation types and VAT categories

- Added validation types for EN16931 compliance in `validation.types.ts`, including interfaces for `ValidationResult`, `ValidationOptions`, and `ValidationReport`.
- Introduced `VATCategoriesValidator` in `vat-categories.validator.ts` to validate VAT categories according to EN16931 rules, including detailed checks for standard, zero-rated, exempt, reverse charge, intra-community, export, and out-of-scope services.
- Enhanced `IEInvoiceMetadata` interface in `en16931-metadata.ts` to include additional fields required for full standards compliance, such as delivery information, payment information, allowances, and charges.
- Implemented helper methods for VAT calculations and validation logic to ensure accurate compliance with EN16931 standards.

assets/schematron/EN16931-EDIFACT-v1.3.14.meta.json

@@ -0,0 +1,7 @@
+{
+  "source": "EN16931-EDIFACT",
+  "version": "1.3.14",
+  "url": "https://github.com/ConnectingEurope/eInvoicing-EN16931/raw/master/edifact/schematron/EN16931-EDIFACT-validation.sch",
+  "format": "CII",
+  "downloadDate": "2025-08-11T11:05:40.547Z"
+}