Convert tthe gem to frozen string literals

While running tests for sorbet/sorbet#7778
I noticed some deprecation warnings coming from this gem.

Ruby 3.4 will have chilled string literals, as a first step toward
frozen string literals becoming the default.

Ref: https://bugs.ruby-lang.org/issues/20205

Gemfile

@@ -1,2 +1,3 @@
+# frozen_string_literal: true
 source 'https://rubygems.org'
 gemspec

Rakefile

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'bundler/gem_tasks'
 require 'bundler/setup'
 require 'rake/testtask'

lib/subprocess.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'thread'
 require 'set'
 
@@ -428,7 +429,7 @@ def drain_fd(fd, buf=nil)
     def communicate(input=nil, timeout_s=nil)
       raise ArgumentError if !input.nil? && @stdin.nil?
 
-      stdout, stderr = "", ""
+      stdout, stderr = +"", +""
 
       # NB: Always force encoding to binary so we handle unicode or binary input
       # correctly across multiple write_nonblock calls, since we manually slice
@@ -508,7 +509,7 @@ def communicate(input=nil, timeout_s=nil)
 
           if block_given? && !(stderr.empty? && stdout.empty?)
             yield stdout, stderr
-            stdout, stderr = "", ""
+            stdout, stderr = +"", +""
           end
         end
       end

lib/subprocess/version.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Subprocess
   VERSION = '1.5.6'
 end

subprocess.gemspec

@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 lib = File.expand_path('../lib', __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'subprocess/version'

test/test_subprocess.rb

@@ -1,4 +1,5 @@
 # coding: utf-8
+# frozen_string_literal: true
 require 'rubygems'
 gem 'minitest'
 require 'minitest/autorun'
@@ -436,7 +437,7 @@ def call_multiwrite_script(&block)
 
       it 'preserves encoding  across IO selection cycles with a block given' do
         process = Subprocess::Process.new(['bash', '-c', script], :stdout => Subprocess::PIPE)
-        stdout = ""
+        stdout = +""
 
         process.communicate do |out, _err|
           stdout << out
@@ -450,7 +451,7 @@ def call_multiwrite_script(&block)
         assert_equal(Encoding::UTF_8, message.encoding)
 
         process = Subprocess::Process.new(['cat'], :stdin => Subprocess::PIPE, :stdout => Subprocess::PIPE)
-        stdout = ""
+        stdout = +""
         process.communicate(message) do |out, _err|
           stdout << out
         end