Add new Style/MapCompactWithConditionalBlock cop

[nobuyo]

This cop checks for usage of select or reject
instead of map { ... }.compact.

# bad
array.map { |e| some_condition? ? e : next }.compact

# bad
array.map do |e|
  if some_condition?
    e
  else
    next
  end
end.compact

# bad
array.map do |e|
  next if some_condition?

  e
end.compact

# bad
array.map do |e|
  e if some_condition?
end.compact

# good
array.select { |e| some_condition? }

# good
array.reject { |e| some_condition? }

---

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.

[nobuyo]

Thank you for your review. I've renamed the cop and tweaked the message.

[bbatsov]

One more thing regarding the name - how about simply Style/MapCompact. I think the part about the block in the name is redundant and slightly confusing.

[nobuyo]

I'm fine with that.

@koic How do you think?

[koic]

It may not be ambiguous to user because the department is different between Performance and Style, but I was concerned about the contrast with Performance/MapCompact cop. This cop seems to be more specific to a particular block than Performance/MapCompact cop.
https://docs.rubocop.org/rubocop-performance/1.13/cops_performance.html#performancemapcompact

That may be over-thinking, though...

[nobuyo]

So... Which should we name? I feel like they're both reasonable, but a little too geeky about the difference in Performance and Style.

[bbatsov]

Naming is hard. :-) Let's just go with the current name.