Add `split-preserve-zoom` configuration

[mitchellh]

Discussion: #8447

Add a new config option split-preserve-zoom that is a packed struct that starts with a single option navigation (default false). The semantics are that when navigation is set, zoomed splits will remain zoomed when split navigation (goto_split) is done.

I'm using a packed structure because there are a variety of scenarios we unzoom splits and we may want to add more options later, unclear.

Example:

split-preserve-zoom = navigation