add testt files

ts/classes.decoder.ts

@@ -0,0 +1,18 @@
+import * as plugins from './plugins.js';
+
+/**
+ * A class to convert a given ZUGFeRD XML string
+ * into a structured ILetter with invoice data.
+ */
+export class ZUGFeRDXmlDecoder {
+  private xmlString: string;
+
+  constructor(xmlString: string) {
+    this.xmlString = xmlString;
+  }
+
+  public async getLetterData(): Promise<plugins.tsclass.business.ILetter> {
+    const smartxmlInstance = new plugins.smartxml.SmartXml();
+    return smartxmlInstance.parseXmlToObject(this.xmlString);
+  }
+}