setup: add nix flake

nix package manager users can setup a shell environnement based on a
flake.nix file that describes tools and versions needed to dev and run
the project, this flake contains:
- nodejs 20
- npm
- typescript language server
- angular language service

nix flakes combinated with direnv allow to open a project folder with
automatic setup launch, creates en .envrc file and a .direnv directory.
-> not every contributors use it and want to have those dotfiles, so it
should be ignored by git.

Co-authored-by: Uriel-Sautron <[email protected]>

.gitignore

@@ -47,3 +47,7 @@ testem.log
 Thumbs.db
 src/environments/*.tmp
 *.iml
+
+# direnv
+.envrc
+.direnv

flake.nix

@@ -0,0 +1,36 @@
+{
+  description = "A flake for a TypeScript project";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    { nixpkgs
+    , flake-utils
+    , ...
+    }:
+    flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import nixpkgs {
+        inherit system;
+      };
+      node = pkgs.nodejs_20;
+      nodePackages = pkgs.nodePackages.override {
+        nodejs = node;
+      };
+      angularLanguageServer = pkgs.angular-language-server;
+    in
+    {
+      devShell = pkgs.mkShell {
+        buildInputs = [
+          node
+          nodePackages.npm
+          nodePackages.typescript
+          nodePackages.typescript-language-server
+          angularLanguageServer
+        ];
+      };
+    });
+}