fix: restore TS 2.3 compatibility

Author: Matt Lewis

src/calendar-utils.ts

@@ -761,13 +761,15 @@ export function getDayViewHourGrid({
   return hours;
 }
 
-export enum EventValidationErrorMessage {
-  NotArray = 'Events must be an array',
-  StartPropertyMissing = 'Event is missing the `start` property',
-  StartPropertyNotDate = 'Event `start` property should be a javascript date object. Do `new Date(event.start)` to fix it.',
-  EndPropertyNotDate = 'Event `end` property should be a javascript date object. Do `new Date(event.end)` to fix it.',
-  EndsBeforeStart = 'Event `start` property occurs after the `end`'
-}
+export const EventValidationErrorMessage = {
+  NotArray: 'Events must be an array',
+  StartPropertyMissing: 'Event is missing the `start` property',
+  StartPropertyNotDate:
+    'Event `start` property should be a javascript date object. Do `new Date(event.start)` to fix it.',
+  EndPropertyNotDate:
+    'Event `end` property should be a javascript date object. Do `new Date(event.end)` to fix it.',
+  EndsBeforeStart: 'Event `start` property occurs after the `end`'
+};
 
 export function validateEvents(
   events: CalendarEvent[],