feat: warn on referenced mutated nonstate

[Rich-Harris]

This implements the warning described in #9571 (comment), and would allow us to merge that PR (without the test added in the most recent commit).

I don't love how invasive this change is. I might try another approach that just checks the references directly — in theory it's less efficient (since you need to walk up the path) but in practice I don't think it would matter even a little bit

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🦋 Changeset detected

Latest commit: 95d0570

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Name	Status	Preview	Comments	Updated (UTC)
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[Rich-Harris]

closing in favour of #9669