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

.github/workflows/shellcheck.yml

@@ -26,8 +26,5 @@ jobs:
       - uses: actions/checkout@v4
       - name: Run ShellCheck
         uses: ludeeus/action-shellcheck@master
-        with:
-          severity: error
-          additional_files: "catppuccin.tmux"
         env:
-          SHELLCHECK_OPTS: "-s bash"
+          SHELLCHECK_OPTS: "-a"