implement transpose letters command

lib/ace/commands/default_commands.js

@@ -256,5 +256,9 @@ canon.addCommand({
         env.editor.insert(lang.stringRepeat(args.text  || "", args.times || 1));
     }
 });
+canon.addCommand({
+    name: "transposeletters",
+    exec: function(env, args, request) { env.editor.transposeLetters(); }
+});
 
 });