DEV: Add auto _map extensions for list site settings with no type (#23331)

Followup to eea74e0e32f704f0756202c3f3ae801bcc8ac8bb. Site settings
which are a list without a list_type should also have the _map
extension added which returns an array based on split("|").

For example:

```
SiteSetting.post_menu_map
=> ["read", "like"]
```

spec/lib/site_setting_extension_spec.rb

@@ -870,6 +870,11 @@ RSpec.describe SiteSettingExtension do
       expect(SiteSetting.ga_universal_auto_link_domains_map).to eq(%w[test.com xy.com])
     end
 
+    it "handles splitting list settings with no type" do
+      SiteSetting.post_menu = "read|like"
+      expect(SiteSetting.post_menu_map).to eq(%w[read like])
+    end
+
     it "does not handle splitting secret list settings" do
       SiteSetting.discourse_connect_provider_secrets = "test|secret1\ntest2|secret2"
       expect(SiteSetting.respond_to?(:discourse_connect_provider_secrets_map)).to eq(false)