Closes #16561

Currently, this parallelizes:

• top-level async deriveds
• top-level awaited variables (let foo = await bar, let foo = $state(await bar))
• top-level await expression statements

but we can definitely do more.

This also doesn't "render things as they're ready" yet, but it should improve rendering time.

The current implementation of this relies on the ability to chunk asynchronous work.
For example, in this code...

let foo = await bar;
await baz;
let qux = $derived(await quux);
console.log(foo, qux);
let a = $state(await foo);
let b = $state.raw(await qux);

The async statements can be chunked like so:

let [foo,, qux] = await Promise.all([bar, baz, $.async_derived(async () => await quux)]);
console.log(foo, qux);
let [a, b] = await Promise.all([foo.then($.state), qux.then($.state)]);

(this code is quite different from the actual compiled output, but this is the gist of it)

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