Add support for post process classes

[javierav]

This PR implements TailwindMerge and its configuration. It has been implemented globally so that the cache it incorporates is shared and used by all ClassVariants instances.

[javierav]

@adrianthedev tailwind_merge-0.13.1 requires ruby version < 4.0, >= 3.1 Should we drop support for Ruby 3.0?

[javierav]

I'm not entirely convinced by the proposal of this PR, since we are just integrating with TailwindMerge. I think it would be better to allow something more generic and let the developer decide. What do you think @adrianthedev?

ClassVariants.configure do |config|
  config.process_classes_with do |classes|
     TailwindMerge::Merger.new.merge(classes)
  end
end

[adrianthedev]

Yeah. That could work as well.
We could expose this precise block in the initializer. Maybe commented out for those who don't want to add TailwindMerge as a dependecy.

[javierav]

@adrianthedev ready for review!

[adrianthedev]

I was thinking about generating the initializer uncommented and leave only the process_classes_with option commented out, but I think I like this approach better.
We are not configuring anything with it, we might as well not load it.
Good call!

[adrianthedev]

Awesoem work @javierav!