Setup tool: Do not change theme if theme is already Newspack

[gamebits]

Is your feature request related to a problem? Please describe.
It is possible to fully configure a Newspack theme without having run the Newspack plugin's setup tool ("wizard"). When the setup tool is eventually run, it will switch the theme and write a new revision of the homepage, potentially undoing prior, manual work.

Describe the solution you'd like
If the current theme's slug begins with newspack, the setup tool should skip the above two steps.

Describe alternatives you've considered
The setup tool could offer checkboxes to confirm that the theme should be switched and the homepage revised.

Additional context

p1699293478642709-slack-C013N9S2C7Q