Consider moving terminal ui to its own module

[jvalkeal]

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There's a lot of new code and while it's convenient to have it in a core package a new dependence to reactor is not optimal to have in a core itself. While we could try to make reactor optional it feels that separate module might be better. Not all want to use "UI" features.

At this point we also need to think older flow components as there is a plan to replace core representation layer in those to low level terminal ui components. This means those needs to live in a same module.