In vite-plugin-svelte, it currently helps SvelteKit set the fallback hydrate value. This PR moves the logic into kit itself so we can remove the hardcoded logic.

Returning a promise is supported since Vite 3 so no peer dep bump is needed. In practice this shouldn't break most SvelteKit apps, unless someone is calling sveltekit() programmatically and accessing the plugins individually.

This will likely conflict with #7990

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