feat: allow deriveds to reference their own values

[Rich-Harris]

#17646 got me thinking: why don't we allow deriveds to reference their own values? It's unusual for it to be useful, but there are arguably valid use cases, at least now that deriveds are writable — things like this:

<script>
	var score = $state(0);
	var highscore = $derived(Math.max(score, highscore || 0));
</script>

<p>score: {score}</p>
<p>highscore: {highscore}</p>

<button onclick={() => score += 1}>score a point</button>
<button onclick={() => score = 0}>start new game</button>
<button onclick={() => score = highscore = 0}>reset</button>

That works on this branch, though one test is failing for reasons I don't really understand. Won't invest any time into figuring it out unless we decide that this does in fact make sense.

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🦋 Changeset detected

Latest commit: b199f83

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

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Playground

pnpm add https://pkg.pr.new/svelte@17666