Export `Composite` component

[ciampo]

Feature request

Summary

Make the Composite component a public API

Examples in other libraries

https://ariakit.org/reference/composite

Motivation

The Composite component would allow consumers of Base UI to implement accessible, custom UI widgets (such as listboxes or grids that need to represent only one tab stop in the page — example) in a way that is 100% consistent with other base ui components.

[marbemac]

Yes! In particular we have a bunch of use cases for custom list boxes with keyboard nav, etc.