feat(core): Improve XML processing and error handling for PDF invoice attachments

ts/index.ts

@@ -1,7 +1,15 @@
 import * as interfaces from './interfaces.js';
+import { ZUGFeRDXmlDecoder } from './classes.decoder.js';
+import { ZugferdXmlEncoder } from './classes.encoder.js';
+import { XInvoice } from './classes.xinvoice.js';
 
+// Export interfaces
 export {
   interfaces,
 }
 
-export * from './classes.xinvoice.js';
+// Export main class
+export { XInvoice }
+
+// Export encoder/decoder classes
+export { ZugferdXmlEncoder, ZUGFeRDXmlDecoder }