feat: add railtie initializer to check package manager version

lib/shakapacker/railtie.rb

@@ -3,6 +3,7 @@
 require "shakapacker/helper"
 require "shakapacker/dev_server_proxy"
 require "shakapacker/version_checker"
+require "shakapacker/utils/manager"
 
 class Shakapacker::Engine < ::Rails::Engine
  # Allows Shakapacker config values to be set via Rails env config files
@@ -14,6 +15,12 @@ class Shakapacker::Engine < ::Rails::Engine
  end
  end
 
+ initializer "shakapacker.manager_checker" do
+ if File.exist?(Shakapacker::VersionChecker::NodePackageVersion.package_json_path)
+ Shakapacker::Utils::Manager.error_unless_package_manager_is_obvious!
+ end
+ end
+
  initializer "shakapacker.proxy" do |app|
  if (Shakapacker.config.dev_server.present? rescue nil)
  app.middleware.insert_before 0,