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.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

⚠️ No Changeset found

Latest commit: f872934

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

Playground

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