disable tmux-battery when there is no battery

acpi seems portable enough to detect this

maglev.tmux

@@ -3,13 +3,13 @@ set -e
 
 PLUGINS=$(tmux show-options -g | grep @tpm_plugins)
 
-# Determine whether the tmux-cpu plugin should be installed
+# Determine what plugins we can use
 SHOW_CPU=false
 if [[ $PLUGINS == *"tmux-cpu"* ]]; then
     SHOW_CPU=true
 fi
 SHOW_BATTERY=false
-if [[ $PLUGINS == *"tmux-battery"* ]]; then
+if [[ $PLUGINS == *"tmux-battery"* ]] && [[ $(acpi -b 2> /dev/null) == *"attery"* ]]; then
     SHOW_BATTERY=true
 fi
 SHOW_NET=false