A custom "focus" mode based off .Max() that emulates 4:3

[tokisuno]

If there isn't already a way to do this, I'd like to have a version of the Max layout that takes aspect ratio as a parameter.

I like writing with truezen in Neovim, and I currently use Max with margin = 92, but none of that compares to how it felt writing in 4:3 on my X61.