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

[GauBen]

Closes #13041

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

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[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 6976513b4177d0ba542f776ebdad20065a510a07

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/package	Minor

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[Rich-Harris]

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