[Fix #11957] Add cop Lint/ExplicitPublicAccessModifier

[yoshiyoshiharu]

Fix #11957

This PR adds a Lint/ExplicitPublicAccessModifier.
It warns explicit usage of public which might be surprising for developers accustomed to placing private methods at the bottom of a class.

---

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

The bad case described in the example of this PR can already be detected by Lint/UselessAccessModifier, so I have concerns about the necessity of providing this new cop.

      #   class Test
      #     public
      #
      #     def test; end
      #   end

https://docs.rubocop.org/rubocop/1.64/cops_lint.html#lintuselessaccessmodifier

[yoshiyoshiharu]

@koic
Thank you for having a look!
As mentioned in #11957, using the public access modifier below the private access modifier doesn't be warned.
ClassStructure may detect this; However, if it is disabled (which is default setting), the following example will not be detected.

      #   # bad
      #   class Test
      #     private
      #
      #     def private_method;
      #
      #     public
      #
      #     def public_method; end
      #   end