Add waehrung function for currency formatting

Author: maxmielchen

src/lib/standard/waehrung.js renamed to src/lib/standard/waehrung.ts

@@ -1,5 +1,5 @@
 
-function waehrung(number) {
+function waehrung(number: number): string {
     return number.toLocaleString('de-DE', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
 };