Text wrap through a `TextFormatter` type

[kirawi]

I needed to write a general text wrapper for #2184 because I kept dealing with broken behavior and painful debugging. This should make it possible to unit-test and be usable for hard-wrapping. Since an event is produced by TextFormatter, I think in the future it could also be used to address #411 to insert virtual text between text by having it take an iterator over VirtualTextSpans and returning them as a special event.