fix: add a way to run code when the current reaction finishes updating

[Rich-Harris]

https://github.com/sveltejs/svelte/pull/16239/files#r2190779548

this might be too much, I don't know. personally my inclination would probably be to not worry about this at all (i.e. not bother with teardown either) as it seems like a crazy contrived situation, but maybe that's just me

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