chore: always populate batch_values when updating a derived

[Rich-Harris]

Never really understood this code if I'm honest. It was necessary at one point (it was introduced in #17105, and if you rewind the repo to that PR and remove the if guard, tests fail) but it doesn't seem to be doing anything now, possibly as a result of the scheduling refactor.

And it's contributing to some undesirable behaviour — if we only populate batch_values when we're reading a derived while an effect is running, then we don't populate batch_values when we call update_derived inside is_dirty. In practice what this means is that we execute the derived once when we're checking to see if an effect that depends on it needs to re-run, and then again when we re-run the effect. That's silly!

This PR doesn't fix that by itself. We additionally need is_dirty(d) to return false if batch_values.has(d). But we can't do that yet, because in some circumstances it will lead to a derived incorrectly remaining DIRTY. Needs further investigation, either in this PR or a follow-up.

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Playground

pnpm add https://pkg.pr.new/svelte@17909

[dummdidumm]

mhhhm something tells me this is wrong; my fear is that we will get an undeleteable batch_values state. But I can't reproduce that, so maybe not?
It works unconditionally since #17362, so that PR fixed it differently somehow (asked copilot to do a git bisect in #17912)