Add Nix flake and development shell (#645)

flake.nix

@@ -0,0 +1,20 @@
+{
+  description = "KytyPS5 development shell";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in
+    {
+      devShells = forAllSystems (system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+        in
+        {
+          default = import ./nix/devshell.nix { inherit pkgs; };
+        });
+    };
+}