Only include url_helpers in Preview class when they are defined (#1795)

Without this, we encounter the following error when running
`bundle exec rake docs:build`:

```
Converting YARD documentation to Markdown files.
rake aborted!
NoMethodError: undefined method `routes' for nil:NilClass
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/preview.rb:7:in `<class:Preview>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/preview.rb:6:in `<module:ViewComponent>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/preview.rb:5:in `<top (required)>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/base.rb:11:in `require'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/base.rb:11:in `<top (required)>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/Rakefile:81:in `require'
/Users/rmacklin/src/github.com/ViewComponent/view_component/Rakefile:81:in `block (2 levels) in <top (required)>'
/Users/rmacklin/.asdf/installs/ruby/2.7.5/bin/bundle:23:in `load'
/Users/rmacklin/.asdf/installs/ruby/2.7.5/bin/bundle:23:in `<main>'
Tasks: TOP => docs:build
(See full trace by running task with --trace)
```

docs/CHANGELOG.md

@@ -10,6 +10,10 @@ nav_order: 5
 
 ## main
 
+* Avoid including Rails `url_helpers` into `Preview` class when they're not defined.
+
+    *Richard Macklin*
+
 * Allow instrumentation to be automatically included in Server-Timing headers generated by Rails. To enable this set the config `config.use_deprecated_instrumentation_name = false`.  The old key `!render.view_component` is deprecated: update ActiveSupport::Notification subscriptions to `render.view_component`.
 
     *Travis Gaff*

lib/view_component/preview.rb

@@ -4,7 +4,7 @@
 
 module ViewComponent # :nodoc:
   class Preview
-    include Rails.application.routes.url_helpers
+    include Rails.application.routes.url_helpers if defined?(Rails.application.routes.url_helpers)
     include ActionView::Helpers::TagHelper
     include ActionView::Helpers::AssetTagHelper
     extend ActiveSupport::DescendantsTracker