feat(darwin): add sketchybar to yabai module

modules/nix-darwin/yabai/sketchybar/plugins/volume.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+# The volume_change event supplies a ${INFO} variable in which the current volume
+# percentage is passed to the script.
+
+VOLUME=${INFO}
+
+case ${VOLUME} in
+  [6-9][0-9]|100) ICON="􀊩"
+  ;;
+  [3-5][0-9]) ICON="􀊥"
+  ;;
+  [1-9]|[1-2][0-9]) ICON="􀊡"
+  ;;
+  *) ICON="􀊣"
+esac
+
+sketchybar --set ${NAME} icon="${ICON}" label="${VOLUME}%"