Accept `var` in place of `VAR` as unit for volt-ampere reactive, as specified by IEC and SI

[adrfantini]

Describe the bug
var, according to the IEC, is the correct unit for volt-ampere reactive, not VAR. The latter is accepted by MathJS, while the former is not!
See here for links and info.

To Reproduce

(new Unit(1, 'VAR')).format() // works. It should not, or only as an alias
(new Unit(1, 'var')).format() // fails. It should!