Test on ruby 4

Author: jcoyne

.github/matrix.json

@@ -7,6 +7,12 @@
   "additional_engine_cart_rails_options": ["--css=bootstrap"],
   "additional_name": [""],
   "include": [
+    {
+      "ruby": "4.0",
+      "rails_version": "8.1.1",
+      "additional_engine_cart_rails_options": "--css=bootstrap",
+      "additional_name": "| Ruby 4"
+    },
     {
       "rails_version": "8.1.1",
       "additional_engine_cart_rails_options": "--css=bootstrap --js=esbuild",