Add new Performance/UnfreezeString cop

[pocke]

In Ruby 2.3 or later, String#+@ is available.
This method unfreezes a string.

str = 'foo'.freeze
p str.frozen?    # => true
p (+str).frozen? # => false

String#dup works similarly, but +@ is faster than dup.
See. https://gist.github.com/k0kubun/e3da77cae2c132badd386c96f2de5768

This cop recommends to use +@ instead of dup.

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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).
• Used the same coding conventions as the rest of the project.
• 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.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[koic]

I think it would be good to add String.new("") to a bad case. It takes an empty string as an argument.

[Drenmi]

It seems like this isn't actually checked by the cop. 🙂

[Drenmi]

"Use" with lowercase "u", since it's in the middle of a sentence. 😊

[Drenmi]

Hm. Where does the plus go here? 🤔

[pocke]

+<<TEXT
  foo
  bar
TEXT

[Drenmi]

Cool! Had no idea this worked. 🙂

[pocke]

Thanks!
I'll add String.new('') to bad case.
And I'll add checks for String.new, String.new('') and String.new('something').

[pocke]

I added checks for String.new, and updated the documentation.

[Drenmi]

Good reminder that I should look at adding an "optional" operator. 😅