minor #3831 Adding mb_strlen (ThomasLandauer)

This PR was merged into the 3.x branch.

Discussion
----------

Adding mb_strlen

Commits
-------

19be50ff Adding mb_strlen

doc/filters/length.rst

@@ -12,8 +12,12 @@ it will return the length of the string provided by that method.
 
 For objects that implement the ``Traversable`` interface, ``length`` will use the return value of the ``iterator_count()`` method.
 
+For strings, `mb_strlen()`_ is used.
+
 .. code-block:: twig
 
     {% if users|length > 10 %}
         ...
     {% endif %}
+
+.. _mb_strlen(): https://www.php.net/manual/function.mb-strlen.php