Add mbstring PHP module as requirement

Depending on installed PHP modules, the search could be very slow on phpBB boards, often resulting a timeout. That issue has apparently existed since the version 3.2.0:  https://tracker.phpbb.com/browse/PHPBB3-15043

The solution is installing mbstring PHP module, as explained in the support forums: https://www.phpbb.com/community/viewtopic.php?f=556&t=2466071&start=45#p14976516

However, it's not stated in official requirements (https://www.phpbb.com/support/docs/en/3.2/ug/) and the presence of mbstring is not checked by the installer or the ACP. Unless search is disabled, mbstring is virtually as useful as json and XML modules.