Remove strong dependency check for mbstring

The mbstring functions that are used here can be implemented directly in PHP, (see https://github.com/nicolas-grekas/Patchwork-UTF8/blob/master/class/Patchwork/PHP/Shim/Mbstring.php )
but checking for the extension directly excludes this kind of compatibility implementations from working.
Check mb_substr instead of mb_strlen.
ext-mbstring is a *strong* suggestion.

It allows to use fake implementations of ext/mbstring.