fix: overhaul each blocks

[Rich-Harris]

As noted in #17126, each blocks are a little buggy when there's async stuff involved. #16977 fixed a bunch of stuff around async blocks/branches, but excluded each blocks because, well, they're complicated. But the time has come to deal with it.

As the commit messages suggest this is very WIP — a bunch of tests are failing, and while it fixes part of the problem (item effects and the fallback effect both need to be created immediately inside the block effect, not later upon commit), it doesn't yet fix the problems relating to overlapping async batches. But I thought I'd open the PR in this unfinished state anyway. Pleasingly, so far it's a net reduction in code and complexity.

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pnpm add https://pkg.pr.new/svelte@17150