fix: afterNavigate callback not running after hydration when experimental async is enabled

[PatrickG]

Fixes #14618

If svelte experimental async is enabled we need to wait a microtask(?) between mounting the app and running the after_navigate_callbacks, otherwise the script blocks run after running the after_navigate_callbacks (which means there are no callbacks yet).

I'm not sure if adding a new test app is the right choice. Maybe we should run all tests once with and once without experimental async instead?

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