Add ShortTon Unit

[stiefeljackal]

Currently, the library recognizes the following for "Ton":

• tonne (1000 kg) - Metric System
• imperial ton (1016.0469088 kg) - Imperial System

The imperial ton, also known as long ton, that is currently recognized by the library is one used by Ireland, Commonwealth countries, or those that use Imperial system. However, the quantity value for ton in the United States is different. In the Unite States, the unit ton, also known as short ton, has a quantity value of 907.18474 kg.

Because the library recognizes mass units of "Ton", it should recognize the long name of short ton as well.

Source:

• https://en.wikipedia.org/wiki/Ton
• https://en.wikipedia.org/wiki/Long_ton
• https://en.wikipedia.org/wiki/Short_ton