Add Rails/DurationArithmetic cop

[pirj]

Backed by a proposed ✔️ guideline https://rails.rubystyle.guide/#duration-arithmetic
rubocop/rails-style-guide#282, rubocop/rails-style-guide#293

TODO:

• replace ??? with PR number in changelog file
• add a guideline in rails-style-guide
• assess the percentage of both styles among https://github.com/eliotsykes/real-world-rails apps
• replace StyleGuide with a proper link to the styleguide

---

Before submitting the PR make sure the following are checked:

• 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.
• 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.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[pirj]

1.hour.until(3.days) results in an error.

[pirj]

I personally prefer 3.days.since(Date.yesterday), but sometimes it's more than just two, and:

• it gets unweildy 1.minute.until(3.days.since(Date.yesterday)) vs Date.yesterday + 3.days - 1.munute
• pattern's complexity would skyrocket

[pirj]

Checked autocorrection on real-world-rails repo, worked fine.

23177 files inspected, 48 offenses detected, 48 offenses corrected 

[pirj]

@andyw8 @koic ping

[andyw8]

Why the :: for this example?

[pirj]

Ah, my bad, I thought it's in the cop's examples.

For node pattern matcher, this doc shows that both Time and ::Time are matched.

[pirj]

Thanks for suggestions! Please have another look.

[koic]

Thanks!

[agrobbin]

I'm curious if there was a specific reason that month, months, year, and years were excluded from these duration methods.

[pirj]

I've only checked Numeric and completely forgot about Integer.

Would you like to send a PR to fix this?

A spec to check that all are covered would be nice to have, too.

[agrobbin]

@pirj done! #622