typecheck svelte files in tests

[Rich-Harris]

Adds svelte-check to the check script in packages/kit, which will hopefully help us catch more type errors early (#1316 (comment)).

This doesn't actually fix any of the surfaced errors, but it's low-hanging fruit if anyone wants to work on that

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[Rich-Harris]

Made two changes to types:

• Added an internal NormalizedLoadOutput that mirrors the public LoadOutput, and changed LoadOutput to allow string | Error for loaded.error. An alternative would be to only allow Error
• Referenced Vite/Svelte types inside the Kit types. This means that people only need to reference Kit types in their own apps