Implement NoReturnFromBlock rule

[toddnestor]

Fixes #4064.

This PR adds a NoReturnFromBlock rule that warns when return is called from inside a block. This rule is turned off by default so as not to cause any breaking changes. It can be enabled by setting AllowReturnFromBlock to false in the config.

This rule is important as a return in a block can often cause bugs.

---

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.

[bbatsov]

One issue that I see which such a rule is that it's more or less the same as forbidding the use of procs, as the behavior of return there is the only difference from a lambda.

@rubocop/rubocop-core What do you think this PR?

[bbatsov]

It'd be good to clarify about the nature of the problems that can be caused.

[bbatsov]

I don't see the point of this config, as it's essentially the same as disabling the cop. Also - what about procs? They are essentially blocks for all practical purposes.

[koic]

The bad case described in the example of this PR can already be detected by Lint/NonLocalExitFromIterator, so I have concerns about the necessity of providing this new cop.

      # @example
      #   # bad
      #   items.each do |item|
      #     return if item.nil?
      #     puts item.some_attribute
      #   end

https://docs.rubocop.org/rubocop/1.64/cops_lint.html#lintnonlocalexitfromiterator

[bbatsov]

Ah, great point! I had forgotten about it.

[toddnestor]

The bad case described in the example of this PR can already be detected by Lint/NonLocalExitFromIterator, so I have concerns about the necessity of providing this new cop.

      # @example
      #   # bad
      #   items.each do |item|
      #     return if item.nil?
      #     puts item.some_attribute
      #   end

https://docs.rubocop.org/rubocop/1.64/cops_lint.html#lintnonlocalexitfromiterator

Ah I wasn't aware of this one, let me test that rule out and see if why it didn't catch the bugs we had recently, I'll close this PR if it is indeed redundant.

[dimakura]

The bad case described in the example of this PR can already be detected by Lint/NonLocalExitFromIterator, so I have concerns about the necessity of providing this new cop.

      # @example
      #   # bad
      #   items.each do |item|
      #     return if item.nil?
      #     puts item.some_attribute
      #   end

https://docs.rubocop.org/rubocop/1.64/cops_lint.html#lintnonlocalexitfromiterator

@koic @bbatsov how about return with an expression, which is allowed by Lint/NonLocalExitFromIterator?

items.each do |item|
  return nil if item.nil? # Lint/NonLocalExitFromIterator doesn't complain about this
  # or
  return 'something' if item.nil? # this is also okay

  # ...
end