List View: Allow folding of nested items

[jasmussen]

What problem does this address?

The list view shows the structure of your content. It is present in a few places, it's in the post editor, it works as a persistent sidebar in the site editor, and it exists as a tool for the navigation block.

To better handle nesting, nested blocks should be foldable, so you can collapse deep nesting trees.

What is your proposed solution?

Add a small chevron to the left of blocks that feature nesting:

Note: This change should apply to anywhere we use the List View component.

[priethor]

This issue is very closely related to #26141, it might be worth combining them into one.

[jasmussen]

Very good point. I'd be happy to integrate the contents of this one in #26141?

[jasmussen]

Closing in favor of #26141.