Treeview: no keyboard support

Keyboard support is required, but not implemented (https://www.w3.org/TR/wai-aria-practices/#keyboard-interaction-22).
We decided to take this out of the scope of the accessibility audit. The remaining treeview issues are in #3228.

- [ ] Each parent node contains or owns an element with role group.
Each child node is contained in or owned by an element with role group that is contained in or owned by the node that serves as the parent of that child.

- [ ] Each element with role `treeitem` that serves as a parent node has aria-expanded set to false when the node is in a closed state and set to true when the node is in an open state. End nodes do not have the aria-expanded attribute because, if they were to have it, they would be incorrectly described to assistive technologies as parent nodes.

Start by reviewing issue #3228, which contains the remaining treeview issues, and compare the requirements with the WAI-ARIA keyboard-interaction guidance linked here. Done means implementing the required keyboard behavior and ensuring parent and child treeitems use the specified role group structure and aria-expanded states.

Written by the indexing model from the issue text.