fix: require cookstyle/chefstyle so style checks actually run

[tas50]

The Rakefile requires the standalone chefstyle gem:

begin
  require "chefstyle"
  require "rubocop/rake_task"
  RuboCop::RakeTask.new(:style) { ... }
rescue LoadError
  puts "chefstyle is not available. ..."
end

but chefstyle appears in neither the Gemfile nor the gemspec — the :cookstyle group declares cookstyle and nothing else. The require therefore falls into the rescue LoadError branch, the :style task is never defined, and style checking silently does not happen.

Chefstyle was absorbed into Cookstyle, which ships the rule set as cookstyle/chefstyle (it sets CHEFSTYLE_CONFIG = true and then loads cookstyle). Requiring that path uses the gem that is actually installed. kitchen-pester's Rakefile already does exactly this.

Expect rake style to start reporting offenses that were previously going unchecked.