b4: Add 'textual' library

The new (experimental) TUI-based patch review workflow has this
additional dependency.

https://b4.docs.kernel.org/en/latest/maintainer/review.html#review-tui-based-patch-review-workflow-alpha

Note that for the pip packaging this is an optional dependency, if you
run b4 without it the rest of the functionality will work and when you
run `b4 review tui` it prints:

```
The TUI requires the textual library.
Install it with: pip install b4[tui]
```

So this could potentially be implemented as a separate package called `b4-tui`,
but I don't really see much benefit to that.



<LeftMouse>