[2.x] Fix <Deferred /> in Svelte adapter

[pedroborges]

In issue #2012, the <Deferred /> component was rendering its default slot before the necessary props were updated, leading to errors when accessing properties on undefined objects. To address this, I updated the <Deferred /> component to subscribe to the $page store and use queueMicrotask skip a beat and switch loaded when the props are expected to be updated.

#2036 improves how $page is updated making it more predictable. The approach in this PR resolves the reactivity issue in the <Deferred /> component without modifying the $page store, maintaining compatibility for existing projects.