Allow nesting sub collections in oracle sidebar

[cwegrzyn]

Currently the oracle sidebar flattens collections, but some users prefer to have them nested. We should have an option that allows this.

One thought:

• Instead of a new setting, add this as an option toggle icon similar to the official obsidian sidebars. This would be easy to turn on/off quickly and easy to discover. Since the state would be preserved in the sidebar view state rather than in settings, it would be customizable per workspace.