Extracted from #109549

@kieferrm: I repeatedly tried to use ArrowDown to go from a cell editor to the output below

@isidorn has a similar ask for using Arrow keys to navigate through input/output. Our current flow is partially inspired by Jupyter's mode system, in which you can use Edit mode to navigate/edit cell inputs and Command mode to manipulate cells and execution.

Our current flow is

• Enter/Escape to focus the cell editor or the cell container (list view)
• When the focus is on the cell editor
  • Arrow key will move to previous/next cell editor (if it's markdown cell, it will then switch to editing mode)
• When the focus in on the cell container / list view
  • Arrow key will move to previous/next cell container
• To navigate to outputs
  • you might press Tab multiple times (alt+tab in the editor otherwise it will only insert a \t)
  • or use cmd+ctrl+down (Focus in Active Output), (I just figured out this today)

We may want to see how to improve the default experience and also allow customization.