* Optional id for the tree item that has to be unique across tree. The id is used to preserve the selection and expansion state of the tree item.
*
* If not provided, an id is generated using the tree item's label. **Note** that when labels change, ids will change and that selection and expansion state cannot be kept stable anymore.
*/
id?:string;
/**
* The icon path for the tree item. When `falsy`, it is derived from [resourceUri](#TreeItem.resourceUri).