Set window size in config

[Nemo157]

I want to have my default layout with a :chats pane on the left, but configured layouts get the space split evenly between the splits making it way too big. It looks like this should be pretty easy to support by exposing WindowLayoutDescription::length into the config.

[Nemo157]

I looked into implementing this, but it's not so trivial, WindowLayoutDescription treats None as 0 if any siblings have a length set. It seems modalkit-ratatui needs some enhancements to support having a length defined for just some children of a split and letting the rest flex to the rest of the space.