[#188] Introduce yarn2nix based package definition for frontend

This commit introduces a new yarn2nix based package definition for the
frontend application. By adding this Nix expression, developers can
easily incorporate the project into their nix-shell based environment.

Changes:
- Added a new file `govtool/frontend/default.nix` that defines the
  yarn2nix based package configuration for the frontend application
  "govtool-frontend". This file includes settings for the project name,
  source directory, package.json file, and yarn.lock file wrapped in the
  `mkYarnPackage` function, enabling developers to utilize the project
  seamlessly in their nix-shell based environments.

govtool/frontend/default.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {} }:
+let
+  project = pkgs.mkYarnPackage {
+    name = "govtool-frontend";
+    src = ./.;
+    packageJSON = ./package.json;
+    yarnLock = ./yarn.lock;
+  };
+in
+project