split panes

One feature I really like from iTerm2 is [split panes](https://iterm2.com/documentation/2.1/documentation-one-page.html#:~:text=selected%20this%20way.-,split%20panes,-iTerm2%20allows%20you). In particular, I always split my tabs vertically, with my text editor in the left pane, and ghcid in the right pane. I can currently achieve this with two termonad windows side-by-side, but this breaks down when I have multiple tabs, as I would like switching to a different tab to switch both to the source code of the new tab and to the ghcid of the new tab.

How would I go about implementing this?

No source files, tests, or entry points are named in the issue. Start by reading the iTerm2 split-panes reference and the 27-comment discussion, then establish the intended pane and tab-switching behavior before locating the relevant Termonad code; done means the requested split layout works across multiple tabs.

Written by the indexing model from the issue text.