Svelte 5: $state proxying opt-out mechanism

[Rich-Harris]

Describe the problem

In some cases, the app developer may know that they don't need deep reactivity. To wring out maximum performance, it would be kind to offer a way to opt out of that behaviour.

Describe the proposed solution

let object = $state.raw({
  count: 0
});

// has no effect! `object` properties are not reactive
object.count += 1; 

// works as expected
object = { ...object, count: object.count + 1 };

We would need to add a Symbol to the object passed in to $state.raw, so that it isn't later made reactive if someone does this:

let big_old_bag_of_reactivity = $state({...});
big_old_bag_of_reactivity.object = object;

Alternatives considered

$state.freeze, $state.shallow, etc

Importance

nice to have

[srav001]

Just wondering, wouldn't $state.shallow be more clear to understand than .raw ?

[Conduitry]

After considering a couple other names $state.frozen is the one currently being used in PR #9851.

[benmccann]

Implemented in #9851