Description of the new feature/enhancement

When splitting twice into three panes, I would like the option to split by parent equally.

Instead of:

+-------+---+---+
|       |   |   |
+-------+---+---+

Ideally it would be like this:

+----+----+----+
|    |    |    |
+----+----+----+

Coming from iTerm this makes much more sense to me.

When splitting vertically within a horizontal pane, it would work in the same fashion.

This could be a new shortcut that when splitting, e.g. "split horizontally and keep parent" where all children are resized when a new child is added or removed. (I don't how how it currently works internally, so I'm guessing here.)