fix: invalidate store when mutated inside each block

[dummdidumm]

fixes #10771

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[Rich-Harris]

This works for {#each $checks as checked}, but not {#each identity($checks) as checked}. Then again, that also doesn't work in Svelte 4. Do we care?

[dummdidumm]

Should've added a comment to the code - I noticed quite soon that anything else than a plain $store access in the array doesn't work in Svelte 4, so I vote "good enough".