fix: allow binding to const with spread in legacy mode

[paoloricciuti]

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Closes #13844...initially i wanted to prevent binding when the binder was in legacy mode but this

<script>
    import A from "./A.svelte";
    let x = $state();
</script>

<A
    bind:x
/>
{x}

works and we can't change it without a breaking (svelte 6 wen?) so disallow this

```svelte
<script>
    import A from "./A.svelte";
    let x = $state();
</script>

<A
    bind:x
    {...{}}
/>
{x}

seems weird.

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🦋 Changeset detected

Latest commit: 0dd2125

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