Forward aria-label / aria-labelledby to the role="tree" element

[eiffel205566]

Summary

Allow consumers to set an accessible name on the tree by accepting aria-label
and aria-labelledby as props on <Tree>, and forwarding them onto the
internal role="tree" element rendered by DefaultContainer.

Background

Per the W3C Treeview pattern,
the element with role="tree" should have an accessible name supplied via
aria-label or aria-labelledby. Today react-arborist (3.4.3) does not
expose any prop to do this — TreeProps<T> has no aria-label /
aria-labelledby fields, and DefaultContainer renders
<div role="tree" …> with hardcoded attributes that don't accept extra props.

The same gap exists at the row level: each row gets role="treeitem" plus
aria-level / aria-selected / aria-expanded (great), but no
aria-label / aria-labelledby. Consumers who do not control the row's
visible text DOM end up with treeitems that have no accessible name. Note that
this is workable today via a custom renderRow (which receives node and
attrs), but the tree-container case has no equivalent escape hatch.

Current workaround (what we're forced to do)

Because there is no public API:

// Forward aria-label onto the role="tree" element via DOM mutation
const treeWrapperRef = useRef<HTMLDivElement>(null)

useEffect(() => {
  const tree = treeWrapperRef.current?.querySelector('[role="tree"]')
  if (tree && ariaLabel) tree.setAttribute('aria-label', ariaLabel)
}, [ariaLabel])

return (
  <div ref={treeWrapperRef}>
    <Tree {...} />
  </div>
)

This works but:

• is a side-effect that mutates DOM produced by another component,
• can race with arborist's own renders,
• is invisible to React DevTools and to anyone reading the JSX,
• doesn't compose with SSR or strict-mode double-invocation cleanly.

`renderContainer` doesn't help — `DefaultContainer` is not exported, so
overriding it means re-implementing ~230 LOC of keyboard handling
(Backspace delete, Tab/Shift+Tab focus traversal, arrow keys, type-ahead
search, etc.).

Proposed API

<Tree
  data={data}
  aria-label="File explorer"
  // or
  aria-labelledby="file-explorer-heading"
>
  …
</Tree>

Internally, `DefaultContainer` would spread the relevant ARIA attributes onto
its `role="tree"` div:

// DefaultContainer
return (
  <div
    role="tree"
    aria-label={tree.props['aria-label']}
    aria-labelledby={tree.props['aria-labelledby']}
    aria-multiselectable={!tree.props.disableMultiSelection || undefined}
    style={{ height: tree.height, width: tree.width, … }}
    // … existing handlers
  >
    …
  </div>
)

(`aria-multiselectable` would be a useful bonus while we're touching this
element — currently always omitted, even though the tree supports
multi-selection.)

A symmetrical option for rows (`<Tree treeItemAriaLabel={(node) => node.data.name}>`
or just always spreading any `aria-label` from `attrs`) would also be welcome,
though that's already workable via `renderRow`.

Prior accessibility work

Thanks for the previous a11y improvements — #198 (aria-level / role
violations) and #223 (aria-expanded) made the row attributes excellent.
This issue is the natural next step for the container.