HPCC-12349 JSON outputUnicode outputs extra characters

JSON outputUnicode and outputUtf8 are handling byte count vs character count inconsistently when calling appendJSONStringValue.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Actually, I see what you are doing (the fact that we have two overloaded functions called utf8CharLen which do subtly different things confused me).

I think the code probably works, but it's VERY confusing.