Add kilowatt-hour unit to NumberFormat

[johanrd]

Electric vehicles and construction equipment have already started to report energy consumption as kilowatt-hours in their telemetry APIs, not to mention the many applications in the building and power industries. An increasing amount of web applications are going to be developed as tools to help the current electrification of society.

It would be helpful if ecmascript could make developing these web apps a tiny bit easier by providing kilowatt-hour as a supported unit in Intl.NumberFormat.

I understand the concerns about feature creep in suppoerted units (e.g. #607), still adding this as a specific issue.

[johanrd]

closing, as moved to #739