fix(package): use provided tsconfig when calling emitDts

[quentinderoubaix]

Handles second task of https://github.com/sveltejs/language-tools/issues/2355

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Latest commit: b08f930d786d05a4777a15f2bf1a17d68595b715

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

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@sveltejs/package	Patch

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