Use $XDG_CONFIG_HOME for configuration, not $HOME/.dart

[jonasfj]

I just noticed $HOME/.dart/. I think it might be better to use $XDG_CONFIG_HOME, which defaults to $HOME/.config/, instead of using $HOME.

In this case we would create config files in:

• $XDG_CONFIG_HOME/dart/,

And default $XDG_CONFIG_HOME to $HOME/.config/ if it is not defined.
I suspect using $APPDATA instead of $XDG_CONFIG_HOME on Windows is fine.

I'm not sure what to do on Mac, maybe stackoverflow has a suggestion.

Not polluting the $HOME folder is generally nice, afaik ~/.config/ is also used by Chrome, spotify, discord, gcloud and many other judging by my ~/.config/ folder.

CC @jwren

[julemand101]

Looks related to:
#41560

[mraleph]

Duplicate of #41560