Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

lib/document_parser.rb

@@ -1,5 +1,4 @@
 module DocumentParser
-
   def get_document_number_variants(document_type, document_number)
     # Delete all non-alphanumerics
     document_number = document_number.to_s.gsub(/[^0-9A-Za-z]/i, "")
@@ -61,5 +60,4 @@ module DocumentParser
   def dni?(document_type)
     document_type.to_s == "1"
   end
-
 end