fix: destroy each items after siblings are resumed

[Rich-Harris]

This fixes ones of the bugs identified in #17240 (comment). (To be honest I'm astonished that it lasted this long.) Specifically, when you have an each block go from ['a', 'b'] to [], and then to ['a'] while items are still outroing, b will never be removed from the DOM, because the a outro is aborted and the callback that destroys effects is never called.

This solution is admittedly a bit more involved than I initially hoped. I wonder if it would be better to coordinate effect destruction at the batch level, though I didn't pursue that as it would be a breaking change. At least this change imposes no costs on each blocks that don't contain outros.

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Latest commit: e976d40

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Playground

pnpm add https://pkg.pr.new/svelte@17258

[Rich-Harris]

Marked as draft, because I'd like to fix the remaining issues as well if I can