Add flag to `doppler login` CLI command to not prompt to overwrite if already authenticated for scope

This would make it easier in certain contexts to “login only if unauthenticated” in a script—my specific use case is setting up Doppler in a VS Code dev container (I’m running doppler login through the postCreateCommand hook). --overwrite could be made to take a boolean arg, for example, or a --no-overwrite flag could be added. This way, if I wrote

doppler login --scope=/ --no-overwrite

the user would only be prompted if they weren’t already authenticated for the / scope.