feat: migrate reassigned deriveds to $derived

[paoloricciuti]

Since now deriveds are writable we can properly migrate even reassigned labeled statements...i did add a check for the version so that in older versions we migrate to compatible code but maybe that's unnecessary?

Also i still migrate this

$: something = 43;

to state because it's better.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 39fef24

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

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15581-svelte.vercel.app/

[dummdidumm]

👍

[github-actions]

Playground

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