Integrate with `focus.nvim` to split nice

[IndianBoy42]

https://github.com/beauwilliams/focus.nvim. Provides a command FocusSplitNicely which automatically splits according to the size of the current window, and uses the golden ratio. I am hoping to use that for Neoterms splits. I can see it going from two ways:

1. neoterm adds a more flexible way to customize how the split is opened, such that I can tell it to run FocusSplitNicely, and then neoterm launches in that newly created window
2. focus.nvim adds a way to run a command after they create the split, and I tell it to open a neoterm terminal (this would require adding a command on this side to open a terminal in an existing split)