More and more code in Qt uses char16_t instead of QChar, even
QString::Data, so reduce the impedance mismatch with such code and
supply a char16_t overload in parallel to the existing QChar* one.
[ChangeLog][QtCore][QStringDecoder] Added appendToBuffer() overload for
char16_t*, complementing the existing overload taking QChar*.
Task-number: QTBUG-106198
Change-Id: I0cb8ab22c897c14b1318a676f5212cc0cf1b72b7
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>