Replace shell.nix with flake.nix (#610)

## Summary
Simplify and make sure dev dependencies are locked with flakes.

---------

Co-authored-by: Darwin D Wu <darwin67@users.noreply.github.com>

Author: darwin67

.envrc

@@ -1 +1 @@
-use nix
+use flake

flake.lock

@@ -0,0 +1,39 @@
+{
+  description = "Inngest JS/TS SDK";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        corepack = pkgs.stdenv.mkDerivation {
+          name = "corepack";
+          buildInputs = [ pkgs.nodejs_20 ];
+          phases = [ "installPhase" ];
+          installPhase = ''
+            mkdir -p $out/bin
+            corepack enable --install-directory=$out/bin
+          '';
+        };
+
+      in {
+        devShells.default = pkgs.mkShell {
+          packages = [ corepack ];
+
+          nativeBuildInputs = with pkgs; [
+            # Node
+            typescript
+            nodejs_20
+
+            # LSPs
+            nodePackages.typescript-language-server
+            nodePackages.vscode-json-languageserver
+            nodePackages.yaml-language-server
+          ];
+        };
+      });
+}