Composite: export useCompositeStore, add more focus-related props

[ciampo]

What?

Extracted from #63569

Export the useCompositeStore hook from the @wordpress/components package and add focus-related props to Composite and Composite.Item components.

Why?

These APIs are needed in the repository, as discovered while working on #63569

How?

By adding exports, TypeScript definintions, JSDocs, Storybook docs, and README docs.

Testing Instructions

• There shouldn't be any runtime changes for existing Composite consumers
• Open Storybook, make sure that the newly added props are correctly documented
• In a IDE supporting TypeScript, make sure that the newly added props work as expected with autocompetion, and that the associated JSDocs are correct
• Check the Composite README and make sure that the newly added props are properly documented\

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[tyxla]

LGTM 👍

Thanks for splitting it out 🚀