Enable new Layout cops

What does this MR do and why?

The following cops are enabled with their default configuration:

• Layout/LineContinuationLeadingSpace
  • Example: gitlab-org/gitlab!105850 (d8b84d68)
• Layout/LineContinuationSpacing
  • Example: gitlab-org/gitlab!105850 (d8b84d68)
• Layout/LineEndStringConcatenationIndentation
  • Example: gitlab-org/gitlab!105850 (d8b84d68)
• Layout/SpaceBeforeBrackets
  • Example: none ✅

See gitlab-org/gitlab!105850 (merged) to test these cops on gitlab-org/gitlab.

$ bundle exec rubocop --format offenses --only Layout/LineContinuationLeadingSpace,Layout/LineContinuationSpacing,Layou
t/LineEndStringConcatenationIndentation,Layout/SpaceBeforeBrackets

723   Layout/LineEndStringConcatenationIndentation
485   Layout/LineContinuationSpacing
163   Layout/LineContinuationLeadingSpace
--
1371  Total in 433 files

Overall, I think we could use heredocs <<~ in some cases instead.

Contributes to #40.