feat(starship): add nix to prompt

starship/config.toml

@@ -11,6 +11,7 @@ $character\
 """
 
 right_format = """\
+$nix_shell\
 $golang\
 $lua\
 $nodejs\
@@ -19,6 +20,10 @@ $rust\
 $cmd_duration\
 """
 
+[nix_shell]
+disabled = false
+format = '[n(\($name\))](bold blue) '
+
 [character]
 success_symbol = '[>](bold green)'
 error_symbol = '[>](bold red)'