fix: Update $derived.call type declaration

[Not-Jayden]

The current type declaration incorrectly suggests $derived.call() will always return void, instead of the return type of the function it receives.

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[changeset-bot]

🦋 Changeset detected

Latest commit: c0d07e2

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

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svelte	Patch

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[dummdidumm]

Thank you!