xterm: add mainProgram

pkgs/by-name/xt/xterm/package.nix

···
     maintainers = with lib.maintainers; [ nequissimus ];
     platforms = with lib.platforms; linux ++ darwin;
     changelog = "https://invisible-island.net/xterm/xterm.log.html";
+    mainProgram = "xterm";
   };
 }