Add rake task assets:vendor

This task runs before dartsass:build to ensures the required files are
present and that the assets are vendored

lib/tasks/assets_vendor.rake

@@ -0,0 +1,14 @@
+namespace :assets do
+  desc "Copy govuk-frontend assets to vendor assets folder"
+  task vendor: :environment do
+    node_govuk_assets_path = Rails.root.join("node_modules/govuk-frontend/govuk/assets")
+    govuk_assets_path = Rails.root.join("vendor/assets")
+
+    system("yarn --frozen-lockfile")
+    system("yarn cache clean")
+    system("mkdir -p #{govuk_assets_path}/assets")
+    system("cp -r #{node_govuk_assets_path} #{govuk_assets_path}")
+  end
+end
+
+Rake::Task["dartsass:build"].enhance(["assets:vendor"])