Make it easier to get ViewComponent previews into an existing app.

If we want previews to be shown inside existing application layouts, which requires application logic. E.g. url helpers and helper methods like current_user etc. Then we need write a custom controller, and either write all the preview stuff from scratch or copy and paste from ViewComponentsController. If we instead move the content of ViewComponentsController inside a module, then we could just mixin this logic into the application code. Then applications only need something like this:

#app/controllers/view_component_preview_controller.rb
class ViewComponentPreviewController < ApplicationController
  include ViewComponent::PreviewActions
end

#config/application.rb
config.view_component.preview_controller = "ViewComponentPreviewController"

And we are free to use all the application specific stuff that comes with ApplicationController