snacks: enable toggle for UI whick-key menu

[practicalli-johnny]

LazyVim configures which-key with Snacks toggle to visually show the state of each toggle in the which-key menu

• https://github.com/LazyVim/LazyVim
• https://github.com/folke/snacks.nvim/blob/main/docs/toggle.md
• https://github.com/LazyVim/LazyVim/blob/main/lua/lazyvim/config/keymaps.lua#L139 (related keymap definitions)
• https://github.com/LazyVim/LazyVim/blob/main/lua/lazyvim/plugins/ui.lua#L279 - UI config that pulls in safe_keymap_set containg snacks toggle defintions

Replicate in AstroNvim