feat: allow await in components

[Rich-Harris]

This is the PR to go with #15845. Feedback and questions should happen on the discussion rather than here.

Closes #1857, closes #3203, closes #5501

How this works

I'll likely flesh this out in more detail later, but a few quick bullet points:

• state changes are now scoped to a Batch. This was always implied (we would flush effects in a microtask, if a flushSync didn't get there first, so that a++; b++ would result in one flush rather than two) but now it's explicit
• Batch has a process method that is responsible for calling process_effects. Whereas previously this would result in effects running immediately (albeit in two phases, so that e.g. $effect effects happen after other updates), this now only happens for async effects (so that we can know if a state change resulted in async work) and block effects (so that we can know if a newly-created branch has async work, or if newly-destroyed branches can be skipped). All other effects (i.e. user effects, or those that update the DOM) are added to the batch
• if, after flushing everything, the batch is 'settled', we run the remaining effects. Otherwise, those effects will be scheduled when the batch's async deriveds resolve
• batches track the previous values of their sources, so that changes can be momentarily reverted in the case of overlapping batches
• blocks like {#if ...} etc no longer append or remove branches directly. Instead, they do that in a commit callback belonging to the batch, which runs when everything is settled
• we introduce the concept of an async derived, which isn't actually a derived at all — it's a source and an async effect (which is just a normal effect with a flag that lets process_effects run it eagerly) in a trenchcoat. This is because deriveds are lazy and would never update in time
• you create an async derived whenever you have a $derived with an await expression (that isn't inside a function), or whenever you have an await expression in a template effect
• accordingly, $.template_effect now takes a third argument, which is an array of all async deriveds
• control flow blocks ({#if ...} et al) can also contain await expressions, in which case they are wrapped in an $.async block. These simply pass the async derived to the inner logic, so that the blocks themselves don't need to be aware that they're dealing with async values

I'm sure I'm overlooking a ton of stuff but that's as much braindump as I can give right now. Will add comments to more of the source code as and when I can.

WIP

This is not finished; there are bugs, and parts of the code are a little messy. Some stuff could possibly be extracted into a separate PR so that this one is smaller and more focused (e.g. converting boundaries to the Boundary class).

• make preserve_context more accurate
• extract getAbortSignal into separate PR?

---

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Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 77e0e60

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15844-svelte.vercel.app/

[elliott-with-the-longest-name-on-github]

More feedback -- still looking really good

[elliott-with-the-longest-name-on-github]

TODO