feat(package): add --preserve-output flag

[GauBen]

Following #13041

It adds a CLI flag not to rimraf the output directory, --preserve-output, plus tests

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🦋 Changeset detected

Latest commit: 207773d

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

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@sveltejs/package	Minor

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13055-svelte.vercel.app/

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[svelte-docs-bot]

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