feat: add $state.frozen rune

[trueadm]

$state.frozen

Similar to $state, $state.frozen is also declared and can be used in many of the same ways (including on classes). However, the properties of any object and arrays are treated as read-only, and cannot be mutated. So if you intend to use objects as state and you want to mutate their properties and have reactivity work by default, it's recommended you use $state instead.

For the cases where you don't want Svelte's reactivity to apply deeply to state, and for those who might want to have more control over their data structures, you might find $state useful. Furthermore, $state.frozen is ideal for those who want to work with data using immutable patterns rather than mutable patterns.

<script>
	let items = $state.frozen([0]);

	const addItem = () => {
		items = [...items, items.length];
	};
</script>

<button on:click={addItem}>
	{items.join(', ')}
</button>

Objects and arrays passed to $state.frozen will be shallowly frozen using Object.freeze().

[changeset-bot]

🦋 Changeset detected

Latest commit: 34581f6

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

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Name	Status	Preview	Comments	Updated (UTC)
svelte-5-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 27, 2023 7:25am

[Conduitry]

Ref #9762.

[Rich-Harris]

this mostly looks good but left some comments regarding testing.

do we want to use readonly in dev? presumably we don't want people mutating objects inside $state.raw, as it'll just fail confusingly

[trueadm]

Addressed feedback, added more tests. I didn't get around to making the object readonly but hopefully I ticked off the other boxes.

[MrHBS]

Should this be called $state.shallow() instead?

[trueadm]

Update the PR. This is now $state.readonly and we now Object.freeze the object shallowly that is passed in.