fix: Always include root layout/error nodes in manifest

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

Closes #13518.

This optimization doesn't work when an app (or a specific bundle, such as the ones generated for edge functions or when split: true in Vercel) has all prerendered routes. Given that the optimization probably isn't actually saving us much, it's probably not worth it to try to find some convoluted workaround.

TODO: Best way to test? Probably create a new app to guarantee it always has 0 non-prerendered routes.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13522-svelte.vercel.app/

[dummdidumm]

There's a prerendering test app, could we use that somehow?
Also needs a changeset

[changeset-bot]

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Latest commit: c93558f

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