[cli] refactor Output usage

[EndangeredMassa]

On the path towards making the CLI entrypoint more testable, this PR refactors usage of Output to be less interconnected to the system. It is now wrapped by a singleton that can be modified (if necessary) as the program flows.

This allows other contexts to use the same singleton after the initial CLI logic parses the args and updates the values on the singleton.

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Supports: https://linear.app/vercel/issue/ZERO-2713/extract-vercel-cli-entrypoint-into-testable-function