add: nix-shell to build and test on NixOS easily

shell.nix

@@ -0,0 +1,20 @@
+{ pkgs ? import <nixpkgs> {} }:
+  let
+    libPath = with pkgs; lib.makeLibraryPath [
+      libGL
+      libxkbcommon
+      wayland
+    ];
+  in {
+    devShell = with pkgs; mkShell {
+      buildInputs = [
+        cargo
+        rustc
+        rust-analyzer
+      ];
+
+      RUST_LOG = "debug";
+      RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
+      LD_LIBRARY_PATH = libPath;
+    };
+  }