[ruby/irb] respect NO_COLOR environment variable

When `NO_COLOR` is set to any non-nil value, output is not colorized.

See https://no-color.org/

401d0916fe

test/irb/test_init.rb

@@ -67,6 +67,22 @@ module TestIRB
       Process.kill("SIGKILL", status.pid) if !status.exited? && !status.stopped? && !status.signaled?
     end
 
+    def test_no_color_environment_variable
+      orig = ENV['NO_COLOR']
+
+      assert IRB.conf[:USE_COLORIZE]
+
+      ENV['NO_COLOR'] = 'true'
+      IRB.setup(eval("__FILE__"))
+      refute IRB.conf[:USE_COLORIZE]
+
+      ENV['NO_COLOR'] = nil
+      IRB.setup(eval("__FILE__"))
+      assert IRB.conf[:USE_COLORIZE]
+    ensure
+      ENV['NO_COLOR'] = orig
+    end
+
     private
 
     def with_argv(argv)