Update `TreeView.Item` to specify `aria-selected` on element with `role="treeitem"`

[joshblack]

This issue first came up in: #2799, specifically with this violation:

Elements with the ARIA role "treeitem" must have the following attributes defined: aria-selected  jsx-a11y/role-has-required-aria-props

Based on the API document, items with role="treeitem" must convey their state via aria-selected: https://github.com/github/primer/blob/main/apis/tree-view-api.md#communicating-a-node-has-been-focused-to-assistive-technology

This issue should include updating TreeView.Item so that aria-selected is specified on the element with role="treeitem"