fix: disallow TODO errors

[Rich-Harris]

'TODO' is a placeholder error code — we shouldn't actually be shipping these. This PR makes it a type error so that we catch them early

Svelte 5 rewrite

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.

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[changeset-bot]

🦋 Changeset detected

Latest commit: eb0085a

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

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