Fix 2.5 assigned block rescue alignment

[gmcgibbon]

Fixes #6433.

Currently, Layout/RescueEnsureAlignment rejects blocks with rescues that are assigned to a variable. In Ruby 2.5, this is valid:

result = [1, 2, 3].map do |el|
  el.to_s
rescue StandardError => _exception
  next
end

---

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 to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[Edouard-chin]

Changes LGTM, you need to add a link to your GH handle name at the end of the CHANGELOG.md file to make the test pass.

There is also an edge case that I think isn't covered by this patch but TBH I'm not even sure what would be the right syntax

if a = something do
  test
rescue => e
  puts 'hello'
end; end

[Drenmi]

Thank you! 🙇

[bbatsov]

Thanks!