Fixes a small thing I noticed while looking into #17461 — whenever you have a @const tag after an await, it injects an async thunk that runs before the declaration even if the declaration is directly above — in other words, for something like this...

{#if true}
	{@const a = await 1}
	{@const b = a + 1}

	<p>{b}</p>
{/if}

...we get this unnecessary middle promise:

var promises = $.run([
	async () => a = (await $.save($.async_derived(async () => (await $.save(1))())))(),
+	() => Promise.all([promises[0]]),
	() => b = $.derived(() => $.get(a) + 1)
]);

This PR removes it, and also gets rid of Promise.all in the case where there's only a single promise.

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