Add PrependTrueOptionOnBeforeDestroy cop

[SHinGo-Koba]

There are possibilities that dependent: :destroy causes unexpectedly deletion of associated records even if you set before_destroy callbacks which are expected to be triggered before dependent: :destroy is invoked.

This can occur because dependent: :destroy is one of callbacks as well as before_destroy, so they are run in the order they are defined.
Related Issue was reported and rails guide refers to this operation.

Unexpected deletion of records could be an incident, so I would be happy to prevent it from happening in advance by rubocop-rails.

Adding prepend: true option on before_destroy is known as one of solutions of it.
So I tried to apply this solution as a new rule.

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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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.