added nix shell for godot mono 4.x

godot-mono/README.md

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+# Godot Mono
+This nix shells builds a shell in which build of Godot engine can be run. Target godot versions are mono 4.x.
+In order to run godot, download the binary from their site, open the nix shell, then run godot.

godot-mono/default.nix

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+{ pkgs ? import <nixpkgs> {} }:
+(pkgs.buildFHSEnv {
+    name = "environment to run mono godot";
+    targetPkgs = pkgs: (with pkgs; [
+        mono6
+        xorg.libX11
+        xorg.libXcursor
+        xorg.libXrandr
+        fontconfig
+        dbus
+        libxkbcommon
+        xorg.libXext
+        xorg.libXinerama
+        xorg.libXi
+        libGL
+        vulkan-loader
+        libpulseaudio
+        dotnet-sdk
+    ]);
+    multiPkgs = pkgs: (with pkgs; [
+        udev
+        alsa-lib
+    ]);
+    runScript = "bash";
+}).env
+