Only add default web dyno if `config.ru` exists

[dentarg]

You could have an app that is not an web app, but for whatever reasons, it has rack as a dependency. Then, the buildpack would automatically add a web dyno to that app when it is deployed for the first time (maybe you do some automated testing with Terraform)

heroku-buildpack-ruby/lib/language_pack/rack.rb

Lines 9 to 11 in 104fe3a

	def self.use?
	bundler.gem_version('rack')
	end

I discussed changing this with @schneems at https://ruby.social/@Schneems/109514562198812396

I don't want to gate on random env vars in general if i can avoid it. I prefer to look for more specific signals if possible.

I would take a PR that gates this behavior on the existance of another file like config.ru considering we use that file in the command:

heroku-buildpack-ruby/lib/language_pack/rack.rb

Lines 23 to 32 in 104fe3a

	def default_process_types
	# let's special case thin here if we detect it
	web_process = bundler.has_gem?("thin") ?
	"bundle exec thin start -R config.ru -e $RACK_ENV -p ${PORT:-5000}" :
	"bundle exec rackup config.ru -p ${PORT:-5000}"
	super.merge({
	"web" => web_process
	})
	end

[schneems]

Thanks a ton for the issue. I would accept this change.