Navigator: internal refactor in preparation for stabilization

[ciampo]

What?

Extracted from #64613
Part of #59418

This PR refactor some internal code of the Navigator component in preparation for the upcoming stabilization.

Why?

See #59418

How?

• Moved current exports from index.ts to a legacy.ts file, which will make room for a new index.ts in a future PR
• Moved all JSDocs to the legacy.ts file, which will allow for a different set of JSDocs to be applied to the stable exports
• Renamed some internal variables "navigator provider" to "navigator", without affecting the exported component name

I don't think a CHANGELOG entry is necessary.

Testing Instructions

• No runtime changes
• Project builds, tests pass
• Smoke test Storybook and block editor, everything should work as before

[github-actions]

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Co-authored-by: ciampo <[email protected]>
Co-authored-by: tyxla <[email protected]>

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[tyxla]

LGTM 👍

Thanks for moving it to a separate PR, it does help a lot with reviewing 🚀