fix: workaround for minification and classnames check

Author: amirhmoradi

Diff for: src/calendarSystems/CalendarSystemBase.js

@@ -14,6 +14,8 @@
 // "iso8601", "japanese", "persian", "roc", "islamicc".
 
 export default class CalendarSystemBase {
+    static typeName = 'CalendarSystemBase';
+    
     constructor(locale = 'en') {
         this.locale = locale;
     }

Diff for: src/calendarSystems/GregoryCalendarSystem.js

@@ -12,7 +12,6 @@
 import CalendarSystemBase from './CalendarSystemBase';
 
 export default class GregoryCalendarSystem extends CalendarSystemBase {
-
      convertFromGregorian(date) {
         // extract year, month, day from date.
         // date can be of type Date, Dayjs or object.

Diff for: src/index.js

@@ -39,9 +39,9 @@ export default (options, dayjsClass, dayjsFactory) => {
 
   // Register a new calendar system
   dayjsFactory.registerCalendarSystem = (name, calendarSystem) => {
-    if (!(calendarSystem instanceof CalendarSystemBase)) {
-        throw new Error('Calendar system must extend CalendarSystemBase');
-      }
+    if (calendarSystem.constructor.typeName !== 'CalendarSystemBase') {
+      throw new Error('Calendar system must extend CalendarSystemBase');
+    }
 
     calendarSystems[name] = calendarSystem;