[IMP] pos_discount: possibility to redefine the discount button

Was not able to make inheritance of  POS Discount objects via "extend" or
"include" functions

Closes #25227

addons/pos_discount/static/src/js/discount.js

@@ -58,5 +58,8 @@ screens.define_action_button({
     },
 });
 
+return {
+    DiscountButton: DiscountButton,
+}
 
 });