mruby-string-ext (append_as_bytes): new method introduced by CRuby 3.4

Since mruby does not have Encoding support, append_as_bytes works
exactly same as String#concat.

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -439,6 +439,8 @@ class String
     self
   end
 
+  alias append_as_bytes concat
+
   def __upto_endless(&block)
     len = self.length
     # both edges are all digits