Support XDG_CONFIG_DIRS

[LelouBil]

So issue #714 got closed without actually implementing XDG_CONFIG_DIRS.

Would it be possible to implement it ?

Building a custom shell that uses Waybar would be a bit easier.

[Vladimir-csp]

I'd also recommend extending fallback paths back to [${XDG_DATA_DIRS}]/waybar/ and put vendor/package config there, leaving [${XDG_CONFIG_DIRS}]/waybar for local admin overrides.