Add `RSpec/ClassCheck` cop

[r7kamura]

It would be nice to have a new cop similar to the Style/ClassCheck cop.

• https://docs.rubocop.org/rubocop/cops_style.html#styleclasscheck

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Updated documentation.
• [x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• [x] Added the new cop to config/default.yml.
• [x] The cop is configured as Enabled: pending in config/default.yml.
• [x] The cop is configured as Enabled: true in .rubocop.yml.
• [x] The cop documents examples of good and bad code.
• [x] The tests assert both that bad code is reported and that good code is not reported.
• [x] Set VersionAdded in default/config.yml to the next minor version.

If you have modified an existing cop's configuration options:

• [ ] Set VersionChanged in config/default.yml to the next major version.