Add new Performance/StringIdentifierArgument cop

[koic]

This PR adds new Performance/StringIdentifierArgument cop.

This cop identifies places where string identifier argument can be replaced by symbol identifier argument. It prevents the redundancy of the internal string-to-symbol conversion.

This cop targets methods that take identifier (e.g. method name) argument and the following examples are parts of it.

# bad
send('do_something')
attr_accessor 'do_something'
instance_variable_get('@ivar')

# good
send(:do_something)
attr_accessor :do_something
instance_variable_get(:@ivar)

The send method is a representative and other similar methods are similar benchmarks. The following is an example benchmark.

% cat symbol.rb
puts `ruby -v`

require 'benchmark/ips'

def foo
end

Benchmark.ips do |x|
  x.report('symbol arg') { send(:foo) }
  x.report('string arg') { send('foo') }

  x.compare!
end

% ruby symbol.rb
ruby 3.1.0dev (2021-12-05T10:23:42Z master 19f037e452) [x86_64-darwin19]
Warming up --------------------------------------
          symbol arg     1.025M i/100ms
          string arg   590.038k i/100ms
Calculating -------------------------------------
          symbol arg     10.665M (± 1.5%) i/s -     53.318M in 5.000551s

          string arg      5.895M (± 1.0%) i/s -     29.502M in 5.004999s

Comparison:
          symbol arg: 10665035.8 i/s
          string arg:  5895132.3 i/s - 1.81x  (± 0.00) slower

I learned about this performance difference from the book "Polished Ruby Programming".

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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.