Support moving between *sections* of the "themes" menu

[ELLIOTTCABLE]

Issue Type: Feature Request

On macOS, search fields often have ‘sections:’

More importantly, in all such sections across the operating system, there's a consistent set of keybindings for navigating these sections. For instance, in the above interface, as an element within the Other section is highlighted, hitting ⌘↓ would ‘leap’ to the first item of the Folders section, regardless of which element in Other was currently selected.

Meanwhile, in VScode, there are several such interfaces. Notably, for me, is the "Themes" interface:

Unfortunately, if my cursor is ‘high up’ in the Light themes section, moving to the Dark themes section is very costly to my eyes, and not quick at all — in fact, it's one of the only things I reliably use my mouse for, every single day!

tl;dr: I'd love keybinds in the list.* category to jump to the first item in the (previous/next) section to the one that the cursor is currently inside. (=

VS Code version: Code 1.32.3 (a3db5be, 2019-03-14T22:46:48.547Z)
OS version: Darwin x64 18.2.0