Follow XDG Base Directory Specification
To comply with XDG Base Directory Specification, the default location for `DATA_DIR` should be `"$XDG_DATA_HOME"/atomic-swap/{ENV}` and `XDG_DATA_HOME` should fallback to `"$HOME"/.local/share`. Some of the contents of `DATA_DIR` could also belong in `XDG_STATE_HOME`, `XDG_CACHE_HOME`, or `XDG_CONFIG_HOME`. The full specification can be found [here](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html).