Programmatically expand/collapse TreeView nodes (#850)

* feat(tree-view): add accessors to programmatically expand/collapse nodes

* feat(tree-view): update docs/types

* test(tree-view): test updated TreeView accessors

* docs(tree-view): document TreeView accessors

types/TreeView/TreeView.d.ts

@@ -66,4 +66,30 @@ export default class TreeView extends SvelteComponentTyped<
     keydown: WindowEventMap["keydown"];
   },
   { labelText: {} }
-> {}
+> {
+  /**
+   * Programmatically expand all nodes
+   * @default () => { expandedIds = [...nodeIds]; }
+   */
+  expandAll: () => void;
+
+  /**
+   * Programmatically collapse all nodes
+   * @default () => { expandedIds = []; }
+   */
+  collapseAll: () => void;
+
+  /**
+   * Programmatically expand a subset of nodes.
+   * Expands all nodes if no argument is provided
+   * @default () => { expandedIds = nodes .filter((node) => !filterNode(node)) .map((node) => node.id); }
+   */
+  expandNodes: (filterId?: (node: TreeNode) => boolean) => void;
+
+  /**
+   * Programmatically collapse a subset of nodes.
+   * Collapses all nodes if no argument is provided
+   * @default () => { expandedIds = nodes .filter((node) => !filterNode(node)) .map((node) => node.id); }
+   */
+  collapseNodes: (filterId?: (node: TreeNode) => boolean) => void;
+}