core(nix/flake): adding flake for developpement on nixos

flake.nix

@@ -0,0 +1,23 @@
+{
+  description = "Flake for 42 C";
+
+  inputs = {
+    flake-utils.url = "github:numtide/flake-utils";
+    c_formatter_42.url = "github:maix-flake/c_formatter_42";
+  };
+  outputs = { self, nixpkgs, flake-utils, c_formatter_42 }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        devShell = pkgs.mkShell {
+          packages = [
+            c_formatter_42.packages.${system}.default
+            pkgs.clang
+            pkgs.clang-tools
+            pkgs.fastmod
+            pkgs.norminette
+            pkgs.tree
+          ] ++ (if pkgs.stdenv.isLinux then [ pkgs.valgrind ] else [ ]);
+        };
+      });
+}