Add support for unicode regular expression flag

The regular expression for matching valid JS regular expressions seems to be missing the `u` flag, which is a valid flag to treat a pattern as a sequence of unicode code points.

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#Advanced_searching_with_flags_2

src/JS.php

@@ -238,7 +238,7 @@ protected function extractRegex()
         // closing the regex)
         // then also ignore bare `/` inside `[]`, where they don't need to be
         // escaped: anything inside `[]` can be ignored safely
-        $pattern = '\\/(?:[^\\[\\/\\\\\n\r]+|(?:\\\\.)+|(?:\\[(?:[^\\]\\\\\n\r]+|(?:\\\\.)+)+\\])+)++\\/[gimy]*';
+        $pattern = '\\/(?:[^\\[\\/\\\\\n\r]+|(?:\\\\.)+|(?:\\[(?:[^\\]\\\\\n\r]+|(?:\\\\.)+)+\\])+)++\\/[gimuy]*';
 
         // a regular expression can only be followed by a few operators or some
         // of the RegExp methods (a `\` followed by a variable or value is