[Feature] Support XDG_CONFIG_HOME and XDG_CONFIG_DIRS

**Please read through the README and the Man pages before submitting**
**Please also make sure that there isn't any prior issue describing this feature**

**Is your feature request related to a problem? Please describe.**
When building custom shells that include swaync, it would be great to simply define XDG_CONFIG_DIRS to the shell's entire config and have swaync look up its configuration there instead of ~/.config

**Describe the solution you'd like**
Support the XDG_CONFIG_HOME and XDG_CONFIG_DIR environment variables, respecting search order

**Describe alternatives you've considered**
Running swaync with -s and -c

**Additional context**

Read the README and man pages first, then trace the existing configuration lookup and the -s/-c options. Confirm the XDG_CONFIG_HOME and XDG_CONFIG_DIRS search order from the issue’s requested behavior; done means swaync finds configuration in those locations instead of only ~/.config while preserving explicit options.

Written by the indexing model from the issue text.