[menu] Add `data-label` attribute to menu items

[benface]

Feature request

Summary

It could be useful for different reasons to add a data-label attribute to the element that Menu.Item (or Menu.CheckboxItem or Menu.RadioItem) renders:

• Easily inspect how Base UI converts children into a string label (when label is not set), e.g. for debugging purposes.
• Write custom search/filtering functionality in the menu (this is what I'm trying to do).
• Maybe even style a specific menu item from an ancestor that doesn't control the rendering of individual items (though it would be a fragile selector).

Examples in other libraries

N/A

Motivation

See above. Thank you!