docs: add $state destructure example using $derived

[anthumchris]

This provides a missing example for destructuring reactive $state objects with $derived. I was unaware this was possible until seeing a related StackOverflow answer. I believe other users will benefit from learning this too.

Test: https://svelte.dev/playground/9f2c5885ac2d4e39a40e69983f7ce8b4?version=5.35.2

[changeset-bot]

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Latest commit: 71a66dd

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16306-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16306

[Rich-Harris]

Thank you but I think it makes more sense to put this information on the $derived page — #16400