[Feature Request]: Support XDG base directory specification

### Feature Description

`expose publish` creates a new configuration file in `~/.expose/config.php`.

This is another directory in my home dir, which gets pretty crowded.
On Linux the [XDG Base Directory Specification](https://specifications.freedesktop.org/basedir/latest/) solves this problem by providing standard directories - including `~/.config/` for configuration files.

It would be nice if expose would follow that specification and store its configuration in `$XDG_CONFIG_HOME` when this environment variable is set, and falls back to `~/.config/` if it is not set.
A fallback to `~/.expose` for existing installations would be necessary as well.

### Is this feature valuable for other users as well and why?

Making backups of one's configuration is easier because users only have to copy `~/.config`.