Cleanup shellcheck warnings (#178)

* chore: shellcheck-warnings for catppuccin.tmux

Command used: `shellcheck catppuccin.tmux -ax -s bash`

* shellcheck: fix all warnings

Command used: `shellcheck -a catppuccin.tmux $(ls custom/*.sh pane/*.sh status/*.sh window/*.sh)`

* shellcheck: update workflow to check warnings

* shellcheck: include warnings from sourced files

builder/module_builder.sh

@@ -36,7 +36,7 @@ build_status_module() {
 
   fi
 
-  if [ $(($index)) -eq 0 ]; then
+  if [ $((index)) -eq 0 ]; then
     local show_left_separator="#[fg=$color,bg=default,nobold,nounderscore,noitalics]$status_left_separator"
   fi