Add textual as a renderer

[melissawm]

This is a work in progress, and the goal is to have feature-parity with papyri browse.

To test, do

$ papyri textual numpy:loadtxt

for example.

[Carreau]

Thanks, that's great.

I think the core of the text we can render only using rich's custom repr formatting, I don't think we can get good keyboard navigation only with rich, and we might need upstream changes.

I'm also thinking of dropping urwid for now, we can always bring it back from the history later.