[Fix #7446] Add merge to NONMUTATING_METHODS

[cstyles]

Fixes #7446

Now, if merge is used in a void context, the Lint/Void cop will
register an offense and the mutating merge! method will be suggested.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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[bbatsov]

Thanks!