chore: use proxies instead of signals for spread/rest props

[Rich-Harris]

Continuation of #9799.

As well as simplifying things internally (yes, it's more code, though I expect it to allow us to delete code in future, hopefully including is_signal which is brittle and ideally wouldn't be necessary), this makes spread/rest props fine-grained in a way that wasn't previously possible.

As an illustration, consider this case:

<script>
	import Child from './Child.svelte';

	let x = $state(0);
	let y = $state(0);
</script>
 
<svelte:window onmousemove={(e) => {
	x = e.clientX;
	y = e.clientY;
}} />

<Child {...{ x, y }} message="hello" />

<script>
	let { ...stuff } = $props();

	function evaluate(value) {
		console.log('evaluating', value);
		return value;
	}
</script>

<p>{evaluate(stuff.message)}</p>

The text effect that renders stuff.message is re-run on every mousemove event even though it's clearly static. On this branch, that doesn't happen.

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• Ideally, include a test that fails without this PR but passes with it.

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Name	Status	Preview	Comments	Updated (UTC)
svelte-5-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 6, 2023 9:04am

[Rich-Harris]

As a result of this PR, the only place we call unwrap is inside each item blocks. That, and bind:this (which feels like an anomaly) are the only places left that deal with the MaybeSignal type

[dummdidumm]

Changes make sense to me 👍 The one caveat is that this iterates over the spread array on each invocation, compared to on each change to the spread before. But since that spread array (the $.spread_array(() => { dynamic: .. }, { static: .. }, ..)) is only one or two elements in the common case it doesn't matter in practise.