fix: make sure moment date formatter is injectable

Author: mattlewis92

projects/angular-calendar/src/modules/common/calendar-moment-date-formatter.provider.ts

@@ -1,4 +1,4 @@
-import { InjectionToken, Inject } from '@angular/core';
+import { InjectionToken, Inject, Injectable } from '@angular/core';
 import {
   CalendarDateFormatterInterface,
   DateFormatterParams
@@ -24,6 +24,7 @@ export const MOMENT: InjectionToken<string> = new InjectionToken('Moment');
  *
  * ```
  */
+@Injectable()
 export class CalendarMomentDateFormatter
   implements CalendarDateFormatterInterface {
   /**