Copied command_exists function to the helpers lib so that there is no hidden dependency on the base plugin

The original command_exists function will stay in the base plugin, but will no longer be used by other plugins or themes.

themes/rana/rana.theme.bash

@@ -117,7 +117,7 @@ ${D_BRANCH_COLOR}%b %r ${D_CHANGES_COLOR}%m%u ${D_DEFAULT_COLOR}"
 
 # checks if the plugin is installed before calling battery_charge
 safe_battery_charge() {
-  if command_exists battery_charge ;
+  if _command_exists battery_charge ;
   then
     battery_charge
   fi