build: add tooling and dev environment configs

Problem: need formatting, linting, testing, and nix dev shell configs.

Solution: add stylua, selene, lua-language-server, busted, pre-commit,
flake.nix, and luarocks rockspec configuration.

flake.nix

@@ -0,0 +1,36 @@
+{
+  description = "task.nvim — oil-like task management 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.luajit.withPackages (
+              ps: with ps; [
+                busted
+                nlua
+              ]
+            ))
+            pkgs.prettier
+            pkgs.stylua
+            pkgs.selene
+            pkgs.lua-language-server
+          ];
+        };
+      });
+    };
+}