Use ES-CN as ISO code for Canary Islands about flag-icons

[NotTsunami]

I'd argue it's not as atypical as you think. #729 mentions using unicode sequences for matching to flag icons and the unicode sequence for Canary Islands is IC. We only strictly conform to ISO 3166-1 alpha-2 codes, though. In the case of the Canary Islands and Ceuta, Melilla, there's no strict specification (source):

The following two areas not covered by European Union Customs arrangements are also exceptionally reserved ISO 3166-1 alpha-2 codes on the request of the World Customs Organization:

EA Ceuta, Melilla
IC Canary Islands
However, these codes do not correspond in any way to the ISO 3166-2 codes of the territories.`

For now, I'd like to leave them as is for users strictly looking for IC. This was also requested multiple times as IC before it officially got added.