Add tmux-like terminal splitting

Would be great for `boo ui` to have vertical and horizontal splits. If libghostty includes this part, that would make it even easier and more seamless of a UX, could imagine taking advantage of the user's existing keybindings here maybe?

I'm really bullish on boo as a modern multiplexer, great job on this. One of the annoying things about using tmux, for example, is the additional UI that it adds on top, so I think boo is a good position to not need this, if libghostty provides all of that.