Rails 8: Include manifest.js in install generator

We found we need to do this for dummy apps, but we also need to do it
for generated apps.

Author: mamhoff

core/lib/generators/solidus/install/install_generator.rb

@@ -121,6 +121,9 @@ def install_routes
 
     def add_files
       template 'config/initializers/spree.rb.tt', 'config/initializers/spree.rb'
+      if Rails.version >= "8.0"
+        template "app/assets/config/manifest.js", "app/assets/config/manifest.js", force: true
+      end
     end
 
     def install_file_attachment

core/lib/generators/solidus/install/templates/app/assets/config/manifest.js

@@ -0,0 +1,3 @@
+//= link_tree ../images
+//= link_directory ../javascripts .js
+//= link_directory ../stylesheets .css