Added Steam.

nodes/hosts/common/programs/steam.nix

@@ -0,0 +1,29 @@
+{ pkgs, ... }: {
+  # Install steam.
+  programs = { 
+    gamemode.enable = true;
+    gamescope.enable = true;
+
+    steam = {
+      enable = true;
+      gamescopeSession.enable = true;
+      remotePlay.openFirewall = true; # Open ports in the firewall for Steam Remote Play
+      dedicatedServer.openFirewall = true; # Open ports in the firewall for Source Dedicated Server
+      localNetworkGameTransfers.openFirewall = true; # Open ports in the firewall for Steam Local Network Game Transfers
+      package = pkgs.steam.override { # Fix Overwatch and other games showing wrong timezone in-game.
+        extraProfile = ''
+          unset TZ
+        '';
+      };
+   
+      protontricks = {
+        enable = true;
+      };
+    };
+  };
+  
+  environment.systemPackages = [
+    pkgs.mangohud
+  ];
+}
+