Add assert.unreachable

[benmccann]

Clear and concise description of the problem

I'm trying to migrate from uvu to vitest and noticed that there's no assert.unreachable

Suggested solution

https://github.com/lukeed/uvu/blob/56e1cb25ecd76cbf5cd3fd28c7a4c07db405c5c3/src/assert.js#L48

Alternative

Just do throw new Error();. Feel free to close this issue if that's preferred

Additional context

No response

Validations

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