Rename cn-xj.svg to east_turkestan.svg

[kylooh]

Rename cn-xj to eastturkestan, because this blue version of moon-star flag is not Xinjiang's flag, but belongs to East Turkestan, a movement or sort of organization. Also there is no provence's flag in PRC.

Acctually, even adding some extra political reasons, in the werstern's view, the relationship between Xinjiang and China, is not that simple like Hong Kong and Taipei. Under that senario, the best solution is also not admitting the East Turkestan not belonging to China.