feat: add atlas to list of proxy executables

[SpotlightKid]

• Also sort list of proxy executables alphabetically

• Fixes No atlas binary in .nimble/bin #80

[SpotlightKid]

I don't think the fix for #80 needs to be any more complicated than that.

I'm not sure what the patch to the original issue dom96/choosenim#333 does, but it it seems to contain many unrelated changes so I didn't use any of it.

With this patch:

• When you switch to a (new or already selected) Nim version with choosenim, the atlas proxy gets written to ~/.nimble/bin.
• It even gets written when the Nim version you switch to doesn't come with atlas, but you won't get an error either.
• When you run the atlas proxy and the currently selected Nim version doesn't have an atlas binary, you'll get the following error:

     Error: Requested executable is missing. (Path: /home/chris/.choosenim/toolchains/nim-1.4.8/bin/atlas)
      Info: If unexpected, please report this error to https://github.com/nim-lang/choosenim

I think this error is sufficiently clear and IMHO it is preferable getting this error than trying to add / remove the atlas proxy depending on whether the selected Nim version comes with atlas or not. This would require checks whether the current atlas binary in ~/.nimble/bin is really a choosenim proxy executable. Otherwise it should not be removed. This check is difficult, because the existing proxy executable may differ from the proxy executable f the currently used choosenim version (i.e. it may have been installed by an earlier choosenim version).