backwards: Enable Steam and let it use the 8BitDo pad

`hid_nintendo` claims the pad when it is in Nintendo Switch mode, which
leaves Steam unable to reach it through hidraw. Blacklist the module and
tag the device with `uaccess` so Steam can open it.

hosts/backwards/modules/games/default.nix

@@ -1,5 +1,13 @@
 { pkgs, ... }:
 {
+  programs.steam.enable = true;
+
+  services.udev.extraRules = ''
+    # 8BitDo in Nintendo Switch mode - allow Steam hidraw access
+    KERNEL=="hidraw*", ATTRS{idVendor}=="057e", ATTRS{idProduct}=="2009", \
+      MODE="0660", GROUP="input", TAG+="uaccess"
+  '';
+
   home-manager.users.alex = {
     home.packages = [
       pkgs.nethack