Fix value subscription on ios and web targets

Hi @arkivanov. I think #59 broke the router for both iOS and the Web targets. I was able to pinpoint the issue down to how the Values are subscribed as State in this extension function

private fun <T : Any> Value<T>.asState(lifecycle: Lifecycle): State<T> {
  val state = mutableStateOf(value)
  observe(lifecycle = lifecycle) { state.value = it }
  return state
}

This works for both Android and Desktop, but not on iOS and Web for some unknown reason.

For now, I was able to fix it temporarily with expect/actual for ios and web to fall back to the previous approach with subscribeAsAState. But this still introduces the issue of the nested router failing to navigate upon returning back.

Before

https://github.com/xxfast/Decompose-Router/assets/13775137/4527a42b-1c93-4e47-a9b9-bfbebbce79de

After

https://github.com/xxfast/Decompose-Router/assets/13775137/2b0f8cde-eb18-4f9d-be0c-8913eaf80771

This PR will

[x] Refactor child stack and navigation out of router
[x] Fix value subscription for ios and web with expect/actual

xxfast / Decompose-Router

Fix value subscription on ios and web targets #64

Before

After