Add g/m2 and mg/m2 to AreaDensity

[Brunni]

According to wikipedia: https://en.wikipedia.org/wiki/Area_density

In the paper and fabric industries, it is called grammage and is expressed in grams per square meter (g/m2)

According to google/wikipedia it is also unofficially abbreviated in fiber as "GSM" (gram per square meter) see https://en.wikipedia.org/wiki/Grammage

Suggestion:

• Add g/m2 to AreaDensityUnit ( https://github.com/angularsen/UnitsNet/blob/master/UnitsNet/GeneratedCode/Units/AreaDensityUnit.g.cs )
• Add "grammage" as a word to the xmldoc for AreaDensity
• As we are currently on it: Also add mg/m² as this is mentioned in the UN CEFACT 20 Standard with common code "GO"