Add floatingPointPrecision option in QJsonDocument

When doing a QJsonDocument::toJson(), the floating point precision for double/float is set to QLocale::[FloatingPointShortest|https://codebrowser.dev/qt6/qtbase/src/corelib/text/qlocale.h.html#QLocale::FloatingPointShortest] in qjsonwriter.valueToJson. wich cause the json to be quite heavy for no particular reason.

So maybe:

int floatingPointPrecision() const; 
void setFloatingPointPrecision(int precision) const; 

can be added to QJsonDocument (with the default value being QLocale::[FloatingPointShortest|https://codebrowser.dev/qt6/qtbase/src/corelib/text/qlocale.h.html#QLocale::FloatingPointShortest] so that it does not affect existing code bases