refactor(aria/tree): remove tree group wrapper

[ok7sai]

This change comes with trade-offs

Pro: This enables a truly flatten tree structure, no wrapper required

<tree>
  <treeitem aria-level="1"></treeitem>
  <treeitem aria-level="2"></treeitem>
  <treeitem aria-level="3"></treeitem>
  <treeitem aria-level="2"></treeitem>
  <treeitem aria-level="1"></treeitem>
</tree>

instead of

<tree>
  <treeitem aria-level="1"></treeitem>
  <treegroup>
    <treeitem aria-level="2"></treeitem>
    <treegroup>
      <treeitem aria-level="3"></treeitem>
    </treegroup>
    <treeitem aria-level="2"></treeitem>
  </treegroup>
  <treeitem aria-level="1"></treeitem>
</tree>

Con: For a nested tree, developers must manually put a role="group" parent to avoid aria-required-parent violation. (treeitem needs a tree or group parent, a treeitem inside a treeitem directly without a group wrapper causes a violation)

<tree>
  <treeitem aria-level="1">
    <treeitem aria-level="2">
      <treeitem aria-level="3"></treeitem>
    </treeitem>
    <treeitem aria-level="2"></treeitem>
  </treeitem>
  <treeitem aria-level="1"></treeitem>
</tree>

above causes violation as group wrappers are required

<tree>
  <treeitem aria-level="1">
    <div role="group">
      <treeitem aria-level="2">
        <div role="group">
          <treeitem aria-level="3"></treeitem>
        </div>
      </treeitem>
      <treeitem aria-level="2"></treeitem>
    </div>
  </treeitem>
  <treeitem aria-level="1"></treeitem>
</tree>