fix: ensure is_pure takes into account runes

[trueadm]

If an expression references a rune, then it's not safe to mark as pure.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14333-svelte.vercel.app/

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[Rich-Harris]

Is this right? If I do {$state.snapshot(1)} — ignore that that's an obviously strange thing to do — then it feels odd that the expression should be marked as impure. Same if I did, I don't know, {$host().nodeName}, and potentially other things we add in future.

Maybe we should just be changing the behaviour of $effect.tracking()? See #13903

[trueadm]

@Rich-Harris Good point. I made that change.