Add DropdownMenu.ItemDescription and DropdownMenu.ItemLabel

[buhodev]

It would be good to include DropdownMenu.ItemDescription and DropdownMenu.ItemLabel components so that users don't have to do aria-describedby and aria-labelledby manually.

Supossing we have this reusable component pseudocode:

<Dropdown>
  <DropdownButton>
    Options
    <ChevronDownIcon />
  </DropdownButton>
  <DropdownMenu>
    <DropdownItem href={url}>
      <DropdownLabel>Open</DropdownLabel>
      <DropdownShortcut keys="⌘O" />
    </DropdownItem>
    <DropdownItem onclick={() => renameFile()}>
      <DropdownLabel>Rename</DropdownLabel>
      <DropdownShortcut keys="⌘R" />
    </DropdownItem>
    <DropdownItem onclick={() => deleteFile()}>
      <DropdownLabel>Delete</DropdownLabel>
      <DropdownShortcut keys="⇧⌘⌫" />
    </DropdownItem>
  </DropdownMenu>
</Dropdown>

then under the hood, <DropdownLabel> will use <DropdownMenu.ItemLabel> and <DropdownShortcut> will use <DropdownMenu.ItemDescription>. This means that each <DropdownItem> will use the ids of <DropdownLabel> and <DropdownShortcut> on the aria-labelledby and aria-describedby attributes automatically.