Implement `<<`

[koppen]

Implementing Classlist on a project and we got

 NoMethodError:
   undefined method `<<' for #<Classlist:0x000000010f544a38 @entries=["col-span-4", "row-span-1", "bg-white", "block-house_ad_component"]>

         super << " p-10 md:p-0"

because we had been treating classes as a String in that case.

Perhaps it makes sense to implement Classlist#<< perhaps as an alias for Classlist#merge?