Fixed typos, renamed routines, added additional tests

Corrected typo: method isSurrogteLead -> isSurrogateLead

Renamed webvtt_utf8_count() to webvtt_utf8_chcount()
Also renamed String::count() to String::charCount()
These changes should help prevent some confusion, because
"length" and "count" seem just about synonymous.

Added some additional tests for working with UTF16
surrogate pairs.