Add each_with_index to Performance/ReverseEach cop

[Babar]

revese.each_with_index has the exact same behavior as reverse.each, therefore add it to the cop too

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Before submitting the PR make sure the following are checked:

• [✅] 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.

[Babar]

Found a computer which could actually run bundle exec rake. Fixed the few mistakes I had in the spec, and now everything looks good.

[Babar]

@koic can you please allow the tests to run so I can confirm this PR is ready? They worked on my machine, but it's only one data point.