fix(core): NG10-style ModuleWithProviders (#2527)

Author: yasinkocak

src/firestore/firestore.module.ts

@@ -9,7 +9,7 @@ export class AngularFirestoreModule {
   /**
    * Attempt to enable persistent storage, if possible
    */
-  static enablePersistence(persistenceSettings?: PersistenceSettings): ModuleWithProviders {
+  static enablePersistence(persistenceSettings?: PersistenceSettings): ModuleWithProviders<AngularFirestoreModule> {
     return {
       ngModule: AngularFirestoreModule,
       providers: [