Add "kvar" unit to reactive power fields

[goxlur]

This adds the unit "kvar" to reactive power measurements.

[paran1]

I am using this to get my power meter working with esphome.
Thanks!

However according to the documentation for the template I think the second unit should be "integer unit", so it should be var/varh not with kilo prefix. For example FixedField, units::kvar, units::kvar); should probably be FixedField, units::kvar, units::var);). This also would require adding var and varh as allowed units.

https://github.com/glmnet/arduino-dsmr/blob/master/src/dsmr/fields.h#L101-L109