feat: out of order rendering

[Rich-Harris]

Extremely WIP. The tests pass, but it's an illusion — there's much still to do.

This will enable out-of-order rendering, which is to say that in a situation like this, where something is awaited in the <script>...

<!-- +layout.svelte -->
<script>
  import { getUser } from '$lib/data.remote';

  let { children } = $props();

  const user = $derived(await getUser());
</script>

<nav>
  {#if user}
    <p>hello {user.name}!</p>
  {:else}
    <a href="/login">log in</a>
  {/if}
</nav>

<main>
  {@render children()}
</main>

...the component's contents can begin rendering immediately — async work in {@render children()} needn't wait for the unrelated async work in the layout.

The basic strategy is this: any statements or declarations containing await — or any statements or declarations that follow them — are wrapped in async functions that run sequentially. So this...

var a = 1;
await sleep();
var b = 2;
await sleep();
var c = 3;

...becomes this:

var a = 1;
var b, c;

var $$promises = $.run([
  () => sleep(),
  () => b = 2,
  () => sleep(),
  () => c = 3
]);

Any parts of the template that depend on b must wait for $$promises[1] to resolve, while any that depend on c must wait for $$promises[3].

Things still to do:

• equivalent of $.run in SSR (right now it's a noop)
• fix some hydration bugs that don't show up in tests but are definitely there
• handle corner cases, like x = $state() followed by x = {...} — anything that depends on x must wait for the latter promise to resolve. This means analysing statements for mutations or function calls that could cause mutations; it may be the case that in some cases we need to deopt and await the final promise
• tidy up the code

Helps with #16561
Also fixes #16978
Solves #17049 (comment) (but possibly not everything in that issue)

Follow-up work:

• reorder statements? So far I didn't do this, because it's potentially dangerous. But it would be nice if we could — for example — yoink $derived expressions as high as they'll go and run them in parallel

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

[dummdidumm]

This looks great! Only a few nits and questions, but overall it seems good.