Vertical layout option for portrait monitors

Instead of infinite horizontal and fixed vertical space, for portrait monitors it makes sense to have infinite vertical and fixed horizontal space. So the same scrolling layout, but going down instead of to the right. And I guess workspaces go sideways then?

Originally requested in: https://github.com/YaLTeR/niri/discussions/757

Unfortunately, this would be quite an annoying and complex refactor in the code.

As an easier alternative, #401 will let you open windows as maximized by default on the portrait monitor.