fix: don't update a focused input with values from its own past

[Rich-Harris]

fixes another lil' regression from #16482 — because we unset current_batch before flushing effects, the 'this input is being updated because of a state change that we started' logic is broken. A simple fix is to track the previous batch, and unset it at an appropriate time

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Latest commit: d5f0b6c

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Playground

pnpm add https://pkg.pr.new/svelte@16491

[Rich-Harris]

working on that no-async test failure