Fixes in word_wrap() functions

Notable points:
* Fix handling of `{unwrap}` tags (d1159ba681a8da618d1b336e3a373a573c68ff87)
* Use `mb_strlen()` in Email library, as in text helper (2737c61e12989832fd901886ffa813b6e24b6a2a)

Just for reference, commits that might be of interest:
* 6ce4746474ddf050f7f4df61b7a22b7f5854d533 (`mb_strlen()` implementation in text helper)
* 9bea4be2efa74b963d3cd0bbaa4635deffc2f109 (one of the very few differences between the 2 functions)