Cope with frozen-string-literal

When running irb 1.2.1 (2019-12-24) with frozen-string-literal enabled, it crashes in reline with `can't modify frozen String (FrozenError)`

Steps to reproduce: 

`RUBYOPT="--enable-frozen-string-literal" irb`

lib/reline/ansi.rb

@@ -172,7 +172,7 @@ class Reline::ANSI
 
   def self.cursor_pos
     begin
-      res = ''
+      res = String.new
       m = nil
       @@input.raw do |stdin|
         @@output << "\e[6n"