fix(units): use var instead of VAR

[adrfantini]

Closes #3613

VAR was kept as alias for backwards compatibility

[gwhitney]

Like "Ah" (#3617),

• Seems straightforward, presuming Wikipedia is correct that var is the official unit name, but I defer to Jos on units.
• Needs work on tests (some are failing, maybe the branch needs syncing?) and both var and VAR need to be tested.
• Needs documentation updated, at least in docs/datatypes/unit.md

One other question: the Wikipedia link says that Var and VAr are common alternate spellings. Should mathjs also accept those?

[adrfantini]

One other question: the Wikipedia link says that Var and VAr are common alternate spellings. Should mathjs also accept those?

Probably, yes, in fact.

I'll update this MR as well as the others as soon as possible, although that might well be a few days or weeks