feat: keybind to jump between trees

Is your feature request related to a problem? Please describe. We have keybinds to jump between parent, child, and sibling nodes, but none to jump between trees, which I often find myself wishing to do.

Describe the solution you'd like Add keybinds Command + Shift + Left and Command + Shift + Right to navigate to trees in the respective directions, like how navigation between sibling nodes works.

Describe alternatives you've considered This is basically a subset of the workspace feature requested in https://github.com/transmissions11/flux/issues/18. That feature is a bit larger and I think we still need a bit more time to decide if that's the optimal way to go, but this specific sub-feature is very tractable and useful even in isolation.