Add new style empty_lines_special to module and class body

[legendetm]

Adds new style empty_lines_special to Style/EmptyLinesAroundClassBody and Style/EmptyLinesAroundModuleBody. It is related to #3452.

It works like empty_lines_except_namespace in #3560, except if the first line is not a method definition, it

• does NOT require an empty line at the beginning of a class/module, but
• requires an empty line before the first method definition.

I'm not sure, whether I should check the empty line before the first method definition in this cop or somewhere else, so currently I made 2 separate commits. If that's fine, I will squash the commits and then this PR fixes #3452.

module Providers
  class ProviderA
    class Xml
      include Provides::Xml

      def to_xml
        ...
      end

    end
  end
end

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[jonas054]

Looks good overall. The only thing I found to comment is that we spell offense with an 's' within the project.

[jonas054]

👍 LGTM. Needs rebase now.