chore: run boundary async effects in the context of the current batch

[Rich-Harris]

We need to rethink how boundaries and batches work in relation to each other, per #16959 (comment). As a starting point, we can get rid of #boundary_async_effects, since separating them out achieves nothing — the distinction is whether the async effect should delay the batch from committing, but that's determined inside the async effect itself, not during traversal, so the only impact of having this array is that the effect runs outside the batch context. But that isn't useful (it doesn't save us from re-running it as necessary), it's just an extra array allocation/iteration.

This will enable further experimentation since we can now guarantee that current_batch exists for all suspendy async effects.

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Latest commit: 7c1408d

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Playground

pnpm add https://pkg.pr.new/svelte@16968

[Rich-Harris]

going to self-merge this to make my local diffs slightly less chaotic