[Feature request] Split horizontally

[Mouradif]

I just discovered this tool while looking for an alternative to less that allows splitting windows horizontally. This repo is the closest to it that I could find.

When debugging the output of a unit test suite I have, I often split my tmux horizontally, run the same unit test in each pane so I can have the outputs side by side and I can scroll each pane to compare their outputs. I wanted a tool to do this without having to run the tests twice (they take a while to run).

I am looking at the code in window.c to see if I could submit a pull request to add this feature