tmux: Underline windows in status line when zoomed

I sometimes forget that windows are in zoomed state as I was missing a
visual indicator. Underline the window names in the status line to fix
that.

TODO: Move added code bit into a variable to prevent redundancy. I was
      not able to get it to be expanded, in contrast it would just
      insert the text literally.

.config/tmux/tmux.conf

@@ -198,8 +198,10 @@ set -g status-left "#{prefix_highlight}"
 
 set -g status-justify centre
 
-set -g window-status-current-format "#[fg=#{@accent_color} bold]#I #W"
-set -g window-status-format "#[fg=#{@ui_color}]#I #W"
+set -g window-status-current-format \
+	"#[fg=#{@accent_color} bold]#{?window_zoomed_flag,#[underscore],}#I #W"
+set -g window-status-format \
+	"#[fg=#{@ui_color}]#{?window_zoomed_flag,#[underscore],}#I #W"
 
 # Initialize TMUX plugin manager. Keep this line at the very bottom.
 run -b "$XDG_CONFIG_HOME/tmux/plugins/tpm/tpm"