Outline: Fold to specific heading level

I merged this into Fold/Unfold headings up to specific level hotkeys and then realized it wasn’t quite the same feature request (it’s asking for the same thing but in the body of the note, not the outline). Sorry for any confusion! When I extracted it back out I gave it a shorter title (was: “Improve Outline core plugin by adding custom tree depth expand/contract controls”).

Related request: