fix(package): use provided tsconfig when calling emitDts

[quentinderoubaix]

Handles second task of sveltejs/language-tools#2355

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🦋 Changeset detected

Latest commit: b08f930

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@sveltejs/package	Patch

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[dummdidumm]

curious: why was this needed?

[quentinderoubaix]

Just to remove a warning while running the unit tests, smth like "tsconfig verbatimModuleSyntax is required when using Svelte files with lang=ts"

This is not related to the original goal of the PR, it can be reverted

[dummdidumm]

Thank you!