add new tree store interface

src/Umbraco.Web.UI.Client/libs/store/tree-store.interface.ts

@@ -0,0 +1,11 @@
+import type { Observable } from 'rxjs';
+
+export interface UmbTreeStore<T> {
+	appendItems: (items: Array<T>) => void;
+	updateItem: (unique: string, item: Partial<T>) => void;
+	removeItem: (unique: string) => void;
+
+	rootItems: () => Observable<Array<T>>;
+	childrenOf: (parentUnique: string | null) => Observable<Array<T>>;
+	treeItems: (uniques: Array<string>) => Observable<Array<T>>;
+}