fix: run effects in pending snippets

[Rich-Harris]

Boundaries are buggy: if the pending snippet contains state, changes to that state won't cause updates:

<script>
	let resolvers = [];

	function push(value) {
		const deferred = Promise.withResolvers();
		resolvers.push(() => deferred.resolve(value));
		return deferred.promise;
	}

	function shift() {
		resolvers.shift()?.();
	}

	let count = $state(0);
</script>

<button onclick={() => count += 1}>
	increment
</button>

<button onclick={shift}>
	shift
</button>

<svelte:boundary>
	<p>{await push('resolved')}</p>

	{#snippet pending()}
		<p>{count}</p>
	{/snippet}
</svelte:boundary>

The issue is that the boundary's this.#effect has the BOUNDARY_EFFECT flag, and this.#pending_effect is a child thereof. Instead, this.#main_effect should have the flag. (It turns out this.#failed_effect also needs the flag, because errors that occur in a failed snippet cause the boundary to re-render in its failed state, which I found somewhat confusing to be honest. Probably the right choice though.)

I was able to simplify the code a bit, too.

(Actually now that I think about it do we need this.#effect at all? Will check.)

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17719

[Rich-Harris]

gah, just realised this will likely conflict with #17672. wanted to get this PR in ahead of working on #16743, which in my head is blocking some other stuff, but we should probably to #17672 first

[dummdidumm]

Mostly looks good but this has the consequence that there's now no more state updates in a failed state: playground

This is arguably worse than the short-lived pending snippet.

If fixing this means removing those lines I added the TODO to, then I think it's reasonable to make this very pedantically strictly speaking breaking change which noone will run into in practise and bubble up an error within a failed snippet to the next boundary (and also add a test to codify this)

Nevermind this was a playground bug not firing events