move `choosenim` under `nim-lang/` official organization

[evan0greenup]

Since choosenim is the officially recommended way to install nim, it would be good to move this repo under https://github.com/nim-lang. It would make it have better future and better integration with nimble.

[evan0greenup]

@dom96 , when considering my this post nim-lang/nimble#1187. It would probably make sense.

[evan0greenup]

After moving this project to nim-lang organization. More contributor will be able to contribute to this project.