fix: properly defer document title until async work is complete

[dummdidumm]

#17061 didn't properly handle the case where the title is sync but reactive and async work outside is pending. Handle this by creating a proper effect for the document title, and make sure to wait on it and flush it once ready.

Fixes #17114

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🦋 Changeset detected

Latest commit: 8549eef

The changes in this PR will be included in the next version bump.

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Playground

pnpm add https://pkg.pr.new/svelte@17158