fix: asynchronously instantiate components when using Svelte 5

[dummdidumm]

Adjusts the behavior to that of the new mount/hydrate APIs and also fixes #12248

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[benmccann]

without the PR title I wasn't sure if this mean sync as in sync/async or as in mirror data. It's also a little funny that it needs to be set it to false. Maybe the API should be something like asynchronous: true?

[dummdidumm]

We can go either way, but I'd go with what we have; this doesn't need to be great in terms of design, it's somewhat of an internal and temporary thing anyway.