Add support for XDG paths?

[Martinsos]

Right now, if you want to use some XDG dir in StrongPath manner, you would do smth like this:

data UserHomeDir
data XdgDataDir

getXdgDataDir :: IO (Path System Abs (Dir XdgDataDir))
getXdgDataDir = fromJust $ parseRelDir $ System.Directory.getXdgDirectory System.Directory.XdgData ""

We could add functions like this directly into StrongPath! Let's say in StrongPath.Xdg module?
Another option, in case we don't want to bloat package with stuff like this, is to put it in a separate package: strong-path-xdg. Or strong-path-extra.

This idea came as a suggestion from @4caraml .