Add `nmi` as an abbreviation for nautical mile

[PesBandi]

Is your feature request related to a problem? Please describe.
Not sure if this counts as a problem, but nmi is not recognized as nautical miles

Describe the solution you'd like
Add nmi to nautical miles

Describe alternatives you've considered
Very simple request, I don't think there are any alternatives

Additional context
Wikipedia mentions nmi.

[PesBandi]

I will open a PR once I read the contributing docs