feat: pass AbortSignal to effects and allow async signature

[trueadm]

WIP

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Latest commit: d2d1b20

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

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svelte	Minor

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pnpm add https://pkg.pr.new/svelte@14753

[trueadm]

It occurred to me that rather than passing in the abort signal as an argument, we could instead capture it using a utility function const signal = getAbortSignal() that you can call inside reactions and then you can pass this to your promises and have the reaction clean it up for you. This also means we can treeshake out this logic if it's not used anywhere.

[briancray]

Besides allowing the async signature (YES PLEASE), how is this different than the following? (I've been doing this pattern myself)

$effect(() => {
  const aborter = new AbortController()
  fetch('/foo', { signal: aborter.signal })
  return () => aborter.abort()
})