Split ui_*() into a UI package

[coatless]

After working with the UI functions, I was impressed by the simplicity and style. As a result, I'm interested in being able to access ui_*() functions without having to add a dependency onto usethis.

Would there be any objection if I extracted these functions and rolled it into a ui package?

[jennybc]

There's a fairly long history of this discussion. #308 should get you on the trail. I think if these ui_*() functions became (part of) a leaner package, it's something we probably want to do -- e.g., coordinate with some other existing packages with related UI & styling functionality.

I'll invite @hadley to chime in here.

[hadley]

I think these will eventually move into the cli package, and you should look there for lighter weight reusable functions.