Separate `dotnetup` interactive / noninteractive modes

[nagilson]

There should be a 'getting started' mode which can set the correct environment variables as in #51124 (the logic for this is not part of this issue, simply calling an interface to do so), looking at the admin installs needed to replicate (if we decide to do so as in #51101) - again this work item can just be an orchestration piece, and then installing the first install. It also needs to enable dotnetup to be called from any terminal directly thereafter, so users can invoke dotnetup install etc afterward. This is the main work in this work item.

dotnetup should also be able to revert the state of modifying the path, and go back to the setup state if needed.