Add split_axis=auto for splits layout #7887

Is your feature request related to a problem? Please describe.

Currently, we can specify the default split axis for the splits layout using either split_axis=horizontal or split_axis=vertical.
However, we can't specify that it should automatically choose whichever direction has the most space.

Describe the solution you'd like
I'd like to configure the split axis direction to be automatic at the enabled_layouts level like this:

enabled_layouts splits:split_axis=auto

Describe alternatives you've considered
The docs suggest mapping a shortcut to:

launch --location=split

This works, and does exactly what I want.
However, I feel it would be nice to be able to specify it in the enabled_layouts configuration, rather than needing to map a shortcut, the same way we can specify split_axis=horizontal, etc.