fix: add SidebarNavHelper to public-api surface

Author: xidedix

projects/coreui/angular/src/lib/sidebar/app-sidebar-nav.service.ts

@@ -13,7 +13,7 @@ export abstract class SidebarNavService {
 @Injectable()
 export class SidebarNavHelper {
 
-  itemType(item) {
+  public itemType(item) {
     if (item.divider) {
       return 'divider';
     } else if (item.title) {

projects/coreui/angular/src/lib/sidebar/public_api.ts

@@ -1,3 +1,4 @@
 export { INavData } from './app-sidebar-nav';
 export { AppSidebarComponent } from './app-sidebar.component';
+export { SidebarNavHelper } from './app-sidebar-nav.service';
 export { AppSidebarModule } from './app-sidebar.module';