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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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14644-svelte.vercel.app/

[hyunbinseo]

If this takes time, could there be a release where SvelteKit warns about using async and afterNavigate together?

It seems to be a better approach then documenting in the Svelte docs.

I have a private library that relies heavily on afterNavigate and it took me quite some time to debug and find this issue.

[korywka]

confirm. if it takes time to merge, please note this bug in remote functions docs or at afterNavigate section as expected. you will save a lot of time for others people for debugging 🙏

[dummdidumm]

I removed the new test suite in favor of running the basic test suite with async mode. I confirmed that a test would fail without this fix; there's another test that fails but that's unrelated to this PR so 👍