nvim: Add markdown todo augend toggle

Use <c-s> and <c-x> to cycle through all todo list item states. Only
works if the cursor is directly on the todo list item.

nvim/.config/nvim/lua/plugins/editing.lua

@@ -36,6 +36,11 @@ return {
 						word = false,
 						cyclic = true,
 					}),
+					augend.constant.new({
+						elements = { "- [ ] ", "- [x] ", "- [o] ", "- [-] ", "- [~] " },
+						word = false,
+						cyclic = true,
+					}),
 				},
 			})
 		end,