fix: Dumber fix for error assertions

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

fixes #13888

We've had issues in the past because isHttpError, isRedirect, and isActionFailure can start reporting false negatives when something somewhere decides to load two copies of the modules the classes are declared in. This switches to a tagged, structural approach that will survive these boundaries. The downside is that some library could throw { _tag: 'SvelteKitRedirect' } and isRedirect would report true, but... I mean, at that point, "hack the library at your own peril", I guess.

This would let us revert our prior attempt at externalizing dependencies, which has caused additional bugs.

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[teemingc]

There's some internal code as well that do not use these helpers and directly uses the instanceof check. We'd have to make changes there too to account for 3rd-party libraries throwing their own instances of these classes.

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Preview: https://svelte-dev-git-preview-kit-13894-svelte.vercel.app/