This has no meaningful functionality yet, it was one of the paths I was
looking at for #8505 but didn't pursue further. But I still think that
this makes more sense in general for the macOS app and will likely be
more useful later.

Fixes #8356

Zoom state should encode as a path so that it can be mapped to a
reference to the node in `root`. Previously, we were encoding a full
node which was instantiating an extra terminal on restore.

This changes equalization so it only counts children oriented in the
same direction.

This makes splits a bit more aesthetic, and replicates how split
equalization works in neovim.

Proper labels, action to move the divider

Fixes #7546

SwiftUI uses type and structure to identify views, which can lead
to issues with tree like structures where the shape and type is the same
but the content changes. This was causing #7546.

To fix this, we need to add explicit identity to the split tree view
so that SwiftUI can differentiate when it needs to redraw the view.

We don't want to blindly add Hashable to SplitTree because we don't want
to take into account all the fields. Instead, we add an explicit
"structural identity" to the SplitTreeView that can be used by SwiftUI.

Fixes regression from #7523

I messed two things up around spatial navigation in the split tree
that this commit fixes:

  1. The distance in the spatial tree only used a single dimension
     that we were navigating. This commit now uses 2D euclidean
     distance from the top-left corners of nodes. This handles the case
     where the nodes are directly above or below each other better.

  2. The spatial slots include split containers because they are layout
     elements. But we should only navigate to leaf nodes. This was
     causing the wrong navigatin to happen in some scenarios.