Add `skip-webpack-install` option

This option is useful when want to check only the files generated by
`rails new`, or if want to do something before `webpacker:install`.

railties/lib/rails/generators/app_base.rb

@@ -399,6 +399,10 @@ module Rails
         !options[:skip_spring] && !options.dev? && Process.respond_to?(:fork) && !RUBY_PLATFORM.include?("cygwin")
       end
 
+      def webpack_install?
+        !(options[:skip_javascript] || options[:skip_webpack_install])
+      end
+
       def depends_on_system_test?
         !(options[:skip_system_test] || options[:skip_test] || options[:api])
       end
@@ -420,7 +424,7 @@ module Rails
       end
 
       def run_webpack
-        unless options[:skip_javascript]
+        if webpack_install?
           rails_command "webpacker:install"
           rails_command "webpacker:install:#{options[:webpack]}" if options[:webpack] && options[:webpack] != "webpack"
         end

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -261,6 +261,9 @@ module Rails
       class_option :webpack, type: :string, default: nil,
                              desc: "Preconfigure Webpack with a particular framework (options: #{WEBPACKS.join('/')})"
 
+      class_option :skip_webpack_install, type: :boolean, default: false,
+                                          desc: "Don't run Webpack install"
+
       def initialize(*args)
         super
 

railties/test/generators/app_generator_test.rb

@@ -825,6 +825,20 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_gem "webpacker"
   end
 
+  def test_skip_webpack_install
+    command_check = -> command do
+      if command == "webpacker:install"
+        assert false, "webpacker:install expected not to be called."
+      end
+    end
+
+    generator([destination_root], skip_webpack_install: true).stub(:rails_command, command_check) do
+      quietly { generator.invoke_all }
+    end
+
+    assert_gem "webpacker"
+  end
+
   def test_generator_if_skip_turbolinks_is_given
     run_generator [destination_root, "--skip-turbolinks", "--no-skip-javascript"]