List View: Explore adding a keyboard shortcut to collapse other list view items

[andrewserong]

What problem does this address?

Part of: #49563

In the list view, when navigating the block hierarchy, it could be useful to have a keyboard shortcut that allows further control over collapsing all other nested groups except the one you have selected.

The problem to solve is that once a large document or template is expanded, it can currently be a bit cumbersome to collapse everything else down except for the thing that you're currently working on.

What is your proposed solution?

@jasmussen brought up the idea of borrowing from the idea in Figma, where ⌥L collapses all other nested groups except the one you have selected. Here's a quick screengrab from Figma:

Note in the above screengrab that the items at the bottom are also collapsed when the item at the top is collapsed.

[andrewserong]

I have a proposed PR for this ready for review over in #59978.