Use `to_prepare` to use autoload inside engine config
Use `to_prepare` to use autoload inside engine config

In Rails 7, previous code raises a NameError.
```
/bundle/ruby/3.0.0/gems/ember-cli-rails-assets-0.6.2/lib/ember_cli/assets/engine.rb:5:in `block in <class:Engine>': uninitialized constant EmberCli::Assets::Engine::EmberCliRailsAssetsHelper (NameError)
```

Using `to_prepare` is recommended to use autoload since Rails 7.
https://github.com/rails/rails/commit/dc047f71b824bfbf4a8e16e2445994d91359d9a2

seanpdoyle/ember-cli-rails-assets,seanpdoyle/ember-cli-rails-assets,seanpdoyle/ember-cli-rails-assets