feature: add Nix Flake support

flake.nix

@@ -0,0 +1,44 @@
+{
+  description = "fvm nix flake";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-24.05";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }: flake-utils.lib.eachDefaultSystem (system: 
+    let
+      pkgs = import nixpkgs { inherit system; };
+      dart = pkgs.dart;
+    in
+    rec {
+      packages = {
+        default = pkgs.stdenv.mkDerivation {
+          pname = "fvm";
+          version = "3.2.1";
+          src = ./.;
+
+          buildInputs = [ dart ];
+
+          buildPhase = ''
+            export PUB_CACHE=$TMPDIR/.pub-cache
+            dart pub get
+	    # avoding codesign issue when building on macOS:
+            export PATH="$PATH:/usr/bin"
+            dart compile exe bin/main.dart -o fvm 
+          '';
+
+          installPhase = ''
+            mkdir -p $out/bin
+            cp fvm $out/bin/
+          '';
+
+          meta = with pkgs.lib; {
+            description = "fvm nix flake";
+            license = licenses.mit;
+          };
+        };
+      };
+    }
+  );
+}