feat(cdk/tree): implement aria-posinset and aria-setsize

Author: BobobUnicorn

src/cdk/tree/tree.ts

@@ -511,6 +511,16 @@ export class CdkTree<T, K = T> implements AfterContentChecked, CollectionViewer,
     return this.treeControl?.getChildren ?? this.childrenAccessor;
   }
 
+  _getPositionInSet(dataNode: T) {
+    const group = this._getNodeGroup(dataNode);
+    return group.indexOf(dataNode) + 1;
+  }
+
+  _getSetSize(dataNode: T) {
+    const group = this._getNodeGroup(dataNode);
+    return group.length;
+  }
+
   private _getAllDescendants(): Observable<T[]> {
     return merge(...(this._dataNodes?.map(dataNode => this._getDescendants(dataNode)) ?? []));
   }
@@ -684,11 +694,11 @@ export class CdkTreeNode<T, K = T> implements FocusableOption, OnDestroy, OnInit
   }
 
   _getSetSize(): number {
-    return 0;
+    return this._tree._getSetSize(this._data);
   }
 
   _getPositionInSet(): number {
-    return 0;
+    return this._tree._getPositionInSet(this._data);
   }
 
   constructor(protected _elementRef: ElementRef<HTMLElement>, protected _tree: CdkTree<T, K>) {