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

spec/lib/document_parser_spec.rb

@@ -2,7 +2,6 @@ require "rails_helper"
 include DocumentParser
 
 describe DocumentParser do
-
   describe "#get_document_number_variants" do
     it "trims and cleans up entry" do
       expect(DocumentParser.get_document_number_variants(2, "  1 2@ 34")).to eq(["1234"])
@@ -26,5 +25,4 @@ describe DocumentParser do
       expect(DocumentParser.get_document_number_variants(1, "1234567A")).to eq(%w[1234567 01234567 1234567a 1234567A 01234567a 01234567A])
     end
   end
-
 end