feat: Setup nixos develop environment

.envrc

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+use flake

flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1787900134,
+        "narHash": "sha256-VYXO0XZlgj06dxJZRhrD3WoSsvq/c7+/Akyoa22pefw=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "83199d0d373dd3ac2b9a1996b1d0263f76ab7a4c",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+  description = "The sandbox programing language purr";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+    in
+    {
+      devShells.${system}.default = pkgs.mkShell {
+        packages = with pkgs; [
+          rustc
+          rust-analyzer
+          rustfmt
+          clippy
+          cargo
+        ];
+      };
+    };
+}