fix: invoke parent boundary of deriveds that throw

[Rich-Harris]

Decided to take a look at our error handling logic. It's rather convoluted and also wrong.

If you have a situation like this...

<!-- App.svelte -->
<script>
  import Component from './Component.svelte';
</script>

<svelte:boundary>
  <Component />
	
  {#snippet failed()}
    <p>outer failed</p>
  {/snippet}
</svelte:boundary>

<script>
  let d = $derived.by(() => {
    throw new Error('kaboom');
  });
</script>

<svelte:boundary>
  {d}
	
  {#snippet failed()}
    <p>inner failed</p>
  {/snippet}
</svelte:boundary>

...the inner boundary fails, even though d is declared outside it. That's obviously incorrect (you can't have boundaries triggering non-deterministically based on where a value happens to be read first). Bit embarrassed I didn't catch this in the initial review.

This PR fixes that behaviour and also simplifies things a decent amount (no more is_throwing_error and should_rethrow_error and previous_effect and other hard-to-follow stuff) and removes some hot path try-catches. It also continues the project of slimming down runtime.js in favour of different modules for different purposes — we're finally under 1,000 LOC.

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

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Latest commit: da2c8a9

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

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