Add new Rails/ToFormattedS cop

[koic]

Follow up rubocop/rails-style-guide#314.

This cop checks for consistent uses of to_fs or to_formatted_s, depending on the cop's configuration.

EnforcedStyle: to_fs (default)

# bad
time.to_formatted_s(:db)

# good
time.to_fs(:db)

EnforcedStyle: to_formatted_s

# bad
time.to_fs(:db)

# good
time.to_formatted_s(:db)

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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.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[andyw8]

Should this check the version of Rails in use and skip if < 7 ?

[koic]

I forgot the minimum_target_rails_version. I've updated this PR. Thank you!