build(flake): add nix devShell

Problem: no nix dev shell existed, making it harder to get local
tooling for formatting, linting, and type checking.

Solution: add a flake with a devShell providing prettier, stylua,
selene, and lua-language-server.

flake.nix

@@ -0,0 +1,30 @@
+{
+  description = "midnight.nvim — a dark colorscheme for Neovim";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    systems.url = "github:nix-systems/default";
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      systems,
+      ...
+    }:
+    let
+      forEachSystem = f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
+    in
+    {
+      devShells = forEachSystem (pkgs: {
+        default = pkgs.mkShell {
+          packages = [
+            pkgs.prettier
+            pkgs.stylua
+            pkgs.selene
+            pkgs.lua-language-server
+          ];
+        };
+      });
+    };
+}