add Rake tasks eslint and lint:javascript

lib/tasks/eslint.rake

+unless Rails.env.production?
+  desc "GitLab | Run ESLint"
+  task :eslint do
+    system("npm", "run", "eslint")
+  end
+end
+

lib/tasks/lint.rake

+unless Rails.env.production?
+  namespace :lint do
+    desc "GitLab | lint | Lint JavaScript files using ESLint"
+    task :javascript do
+      Rake::Task['eslint'].invoke
+    end
+  end
+end
+