Add ability to open / close all items in Outline

[idrisr]

Is your feature request related to a problem? Please describe.
I'm a big fan of the outline feature, but I think it could be improved. My use case is to open the outline to get a high-level overview of the file. The outline view opens by default with all items expanded, and therefore I have to go to each item and toggle it to a closed state with 'o'.

Describe the solution you'd like
I would prefer a way to open the outline with all details not shown
, so that I'm only seeing the top-level items of the outline.

Describe alternatives you've considered
I tried using the typical vim fold commands, but that didn't do what I wanted. Really, what I'm asking for is something like the fold commands, where I can expand everything, collapse everything, or expand/collapse one level, on one particular item.

Additional context
Create some method to easily toggle between these two states