feat: bindings cheatsheet

[bennypowers]

(1) Describe your proposal and benefits: I propose a(n optional?) keybinding, for example Super+? to show/hide an overlay that lists active Forge bindings, a la vim which-key

(2) Describe alternatives you've considered: Improve my memory, perhaps via Omega3 supplements

(3) Design / Screenshots / Mockups:

here's a screenshot of nvim whichkey

here's a screenshot of Black Box' keyboard shortcuts helper window