feat: hydratable and friends

[elliott-with-the-longest-name-on-github]

This is a draft, it's not ready, leave me alone

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[changeset-bot]

⚠️ No Changeset found

Latest commit: d36894a

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16960-svelte.vercel.app/

[paoloricciuti]

This is a draft, it's not ready, leave me alone

[nosovk]

Very interesting approach