feat: support using, link top-level using declarations in components to lifecycle

[Rich-Harris]

Alternative to #16190, with the distinction that using declarations at the top level of a component are only disposed when the component unmounts.

I think I prefer this — it's more useful and more intuitive. Demo here.

On the server it doesn't transform anything, meaning resources are correctly disposed of once rendering is complete. Obviously all this is of limited use until support is sufficiently widespread, but I don't see any reason to hold it back until then. (The demo above actually works in all browsers, because top-level using gets compiled away for client code, but that doesn't apply to server code or non-top-level using.)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8f5a073

The changes in this PR will be included in the next version bump.

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svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16192-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16192