Support Ruby 2.7's pattern matching for Layout/SpaceAroundKeyword

[koic]

This PR supports guard if and unless syntax keywords of Ruby 2.7's pattern matching for Layout/SpaceAroundKeyword.

case 42
in 42 if true
  puts 'hello'
end

These nodes are called if-guard and unless-guard when using guard if and unless in pattern matching.

% ruby-parse example.rb
(case-match
  (int 42)
  (in-pattern
    (int 42)
    (if-guard
      (true))
    (send nil :puts
      (str "hello"))) nil)

See below for this guard semantics:
whitequark/parser#574 (comment)

---

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• 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.

[bbatsov]

Thanks!