Fix Style/BlockDelimiters for blocks with numbered arguments

[gsamokovarov]

With EnforcedStyle of line_count_based, the default, the following code will issue an offense, and will be auto-corrected:

[1, 2, 3].map { |n| # [1, 2, 3].map do |n|
  n + 1             #   n + 1
}                   # end

While the code below won't be converted to a multiline do ... end block:

[1, 2, 3].map {
  _1 + 1
}

This is yet another case of missing on_numblock implementation. I see a dozen of those in the codebase and I think whenever we handle block AST nodes we should also handle numblock nodes. I can go over the codebase and fix those problems on a per-cop basis, or I can try to fix it generally.

Currently, my best idea is to write an InternalAffairs cop that warns when we define on_block handlers, but we miss an on_numblock implementation. For the majority of the cases, on_numblock alias of the on_block method would be enough to handle blocks with numbered arguments. I can then use the internal cop and systematically go over the cases and add aliases or custom numblock handlers where required. I think this is better than solutions involving meta-programming or default on_numblock implementations delegating to on_block.

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• 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]

Thanks!