[Fix #11567] Fix Layout/EndAlignment false negative

[j-miyake]

Fixes #11567

This PR adds the singleton class definition to the inspection targets of Layout/EndAlignment.

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• 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.

[koic]

Thanks!