build: add flake.nix for direnv use

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1707205916,
+        "narHash": "sha256-fmRJilYGlB7VCt3XsdYxrA0u8e/K84O5xYucerUY0iM=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "8cc79aa39bbc6eaedaf286ae655b224c71e02907",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,26 @@
+{
+  description = "nimpkgs website";
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs = inputs @ {
+    self,
+    nixpkgs,
+  }: let
+    inherit (nixpkgs.lib) genAttrs;
+    supportedSystems = ["x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin"];
+    forAllSystems = f: genAttrs supportedSystems (system: f nixpkgs.legacyPackages.${system});
+  in {
+    devShells = forAllSystems (pkgs: {
+      default = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          nim
+          nim-atlas
+          watchexec
+          nodePackages.pnpm
+        ];
+      };
+    });
+  };
+}