Move Spec context state into Spec::CLI

[HertzDevil]

Follow-up to #14170. This includes the global variables inside Spec::RootContext.

[straight-shoota]

thought: I'm wondering if the class name CLI is actually a good fit, particularly with these changes. It seems to be more than a command line interface. Maybe a more generic term like Run might be better?

question: It seems the refactoring from print_results to prints_summary? is a prerequisite but not directly related to the context state refactor. Could we separate this?

[HertzDevil]

CLI is at the moment still :nodoc:, we could change it any time we want.

prints_summary? reflects the fact that formatters don't depend on a context. Any other refactoring would introduce an equally "unrelated" change.

[straight-shoota]

Not sure I understand what you mean. Wouldn't it be possible to apply the refactor from print_results to prints_summary? without the other changes? If it is, we should do it first, then move the other stuff around.