stack should support XDG_CONFIG_HOME/XDG_DATA_HOME

[legrostdg]

It would be great if stack could use XDG_CONFIG_HOME/XDG_DATA_HOME, and $HOME/.stack as a fallback.
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

[mgsloan]

I don't think we want to change this, as it would mean that the stack root would change directories. So the user would be left with an old unused stack root, and a new one.

This may well be more "correct", though. One possible option might be to check for $HOME/.stack first, and if it doesn't exist, also look for $XDG_DATA_HOME/stack, orso.

[damienflament]

See #4243.