Add global option for pixi config, as in `pixi config edit --global`

[corneliusroemer]

Problem description

I might want to edit (and possibly create) a global pixi config file from within a directory that has pixi.

Right now, I don't think I can do this with pixi config edit, without moving out of the directory.

Would be nice if there was a -g/--global flag that would open the global file, rather than the local one.

[ruben-arts]

This already works but you need to add it in the end and with full name.

pixi config edit --global

1586 adds it to the documentation and adds the short version.