Encapsulate stale_when_importmap_changes method

[dhh]

Most Rails applications use etags to some extent. We should therefore make it the default to include the importmap digest in that calculation on HTML requests. So we turn:

class ApplicationController < ActionController::Base
  etag { Rails.application.importmap.digest(resolver: helpers) if request.format&.html? }
end

...into:

class ApplicationController < ActionController::Base
  stale_when_importmap_changes
end

And we can add that add that to be part of the default ApplicationController in Rails when using import maps.