Add Rails/ClassDescendants cop

[alexanderadam]

A wise man, let's call him Ben, once said:

There are two reasons why this is an unreliable solution:

• it doesn't know about things that have yet to be autoloaded

• it's non-deterministic with regards to Garbage Collection of classes. If you use Class.descendants in Test, where there is a pattern to dynamically define classes, GC is unpredictable for when those classes are cleaned up and removed by the GC.

And I totally trust him on that because he's making a… GoodJob 🥁

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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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
  ➡️ Add .descendants and .subclasses advice rails-style-guide#373

fixes #1544

[rafaelfranca]

It is not up to me to say if this should be accepted or not, but Rails provides many sharp knives, this is one of them. Not only #dencendents or #subclasses or really any think that would get inheritance in a Ruby program can see objects that aren't loaded, so I'm not sure this is a valid warning to show users.

Same with Garbage collection. Objects that are GC'ed aren't visible for the Ruby VM anymore.

This is more about knowledge of the Ruby VM than a warning about some specific feature of Ruby or Rails.

[koic]

I agree with @rafaelfranca's opinion. The risks of using #subclasses are already documented as behavior that should be taken into account:
https://docs.ruby-lang.org/en/3.4/Class.html#method-i-subclasses

It might make sense to suggest adding similar information to the #descendants documentation, but that would be up to the Rails core team to decide. Only #descendants (and #subclasses) should not be treated as special cases.

Of course, this represents the stance of RuboCop Rails and does not preclude implementation in a third-party RuboCop gem. Thank you.