Vim Adds XDG_CONFIG_HOME Support

Neovim also has $NVIM_APPNAME which lets one change which directory inside is looked at.

I spent a while a bit back trying different prepackaged neovim setups (before ultimately settling in AstroVim) and it was so checking convenient being able to download one of these setups, stuff it in ~/.config/astrovim or what not, and set NVIM_APPNAME to try it. It made me really see that the XDG directory spec is like half the battle, half the programmability.

Nice to see vim growing into some well established standards. Afaik this whole APPNAME thing doesn't have any write ups, no specs, but would love to see it speced up and propagated further.