minecraft: add forge-impure server

does everything very undeclaratively since nix-minecraft does not
include forge packages.

modules/games/server/minecraft/forge-impure.nix

@@ -0,0 +1,32 @@
+{ config, pkgs, ... }:
+
+let
+  forge-impure = pkgs.vanillaServers.vanilla.overrideAttrs (oldAttrs: {
+    src = ./forge.jar;
+  });
+in
+{
+  services.minecraft-servers.servers.forge-impure = {
+    enable = true;
+    package = forge-impure;
+    autoStart = true;
+    jvmOpts = "-Xms4096M -Xmx4096M";
+
+    serverProperties = {
+      server-port = 30004;
+      difficulty = 3;
+      max-players = 30;
+    };
+  };
+
+  services.minecraft-servers.servers.velocity.symlinks."velocity.toml".value = {
+    servers = {
+      fabric-modless = "127.0.0.1:30004";
+
+      # In what order we should try servers when a player logs in or is kicked from a server.
+      try = [
+          "forge-impure"
+      ];
+    };
+  };
+}