chore: change error helpers to use public interface types

[eltigerchino]

closes https://github.com/sveltejs/kit/issues/12986

This PR changes the error, isHttpError, redirect, and isRedirect functions to use the public interface version of the HttpError and Redirect errors instead of our internal classes.

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[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 9c4eb9699529bc5e22c3654b5863d434d7a5f7c7

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

This PR includes changesets to release 1 package

| Name | Type | | ------------- | ----- | | @sveltejs/kit | Major |

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13036-svelte.vercel.app/

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