starship: Add shell indicators for bash and nu

Added little single-character displays at the front of the prompt if we
are in a shell different from zsh to remind myself (since they all look
exactly the same with starship).

terminal/.config/starship.toml

@@ -82,8 +82,12 @@ style = 'bold yellow'
 format = "[$symbol]($style)"
 
 [custom.shell_bash_indicator]
-command = 'echo 󱆃'
-when = ''' test "$0" = "/usr/bin/bash" '''
+command = 'echo '
+when = ' test "$STARSHIP_SHELL" = "bash" '
+
+[custom.shell_nushell_indicator]
+command = 'echo '
+when = ' test $env.STARSHIP_SHELL "==" "nu" '
 
 # TODO: Would be lovely to have jujutsu support here
 # An example implementation: https://github.com/jj-vcs/jj/wiki/Starship