[simple-mode addon] Don't drop regexp unicode flags

See https://discuss.codemirror.net/t/some-of-my-simple-mode-unicode-patterns-dont-work-why/3101

addon/mode/simple.js

@@ -68,6 +68,7 @@
     var flags = "";
     if (val instanceof RegExp) {
       if (val.ignoreCase) flags = "i";
+      if (val.unicode) flags += "u"
       val = val.source;
     } else {
       val = String(val);