chore(config): clean up and reorganize configuration files

dot_config/tmux/tmux.conf

@@ -104,3 +104,12 @@ set -g message-style "bg=default,fg=blue"
 set -g message-command-style "bg=default,fg=blue"
 set -g mode-style "bg=blue,fg=black"
 setw -g clock-mode-colour blue
+
+# Plugins (tpm, cloned by .chezmoiexternal.toml). prefix + I installs,
+# prefix + U updates, prefix + alt + u removes unlisted ones.
+set -g @plugin 'tmux-plugins/tpm'
+set -g @plugin 'tmux-plugins/tmux-sensible'
+set -g @plugin 'vaaleyard/tmux-dotbar'
+
+# Keep at the very bottom.
+run '~/.config/tmux/plugins/tpm/tpm'