UnitSystem/MapUnitToAbbreviation: Append abbreviations to any existing
UnitSystem/MapUnitToAbbreviation: Append abbreviations to any existing

So GetDefaultAbbreviation() is not affected if an abbreviation is already
defined.

Tirael/UnitsNet,BrandonLWhite/UnitsNet,BoGrevyDynatest/UnitsNet,anjdreas/UnitsNet,neutmute/UnitsNet,BrandonLWhite/UnitsNet,anjdreas/UnitsNet