fix(core): Export VendureEntityEvent abstract class from index (#2556)

packages/core/src/event-bus/index.ts

@@ -1,6 +1,7 @@
 export * from './event-bus';
 export * from './event-bus.module';
 export * from './vendure-event';
+export * from './vendure-entity-event';
 
 export * from './events/account-registration-event';
 export * from './events/account-verified-event';