[Master Stack] Support multiple windows in master pane of master/stack layout

[mwglen]

XMonad has a feature where you can increment/decrement the number of windows that are on the left pane in the master/stack layout. This gives extra flexibility to the layout.

Supporting this in Hyprland would require adding two extra commands: one to increment and one to decrement the count (or one command with a parameter). If the count goes to zero, the left pane vanishes and is replaced by an expanded stack.