Add support for reading the config path from env

[leophys]

Please, have mercy. I am a mere go dev, and this is my first attempt at C++. I wanted to add support to reading the config path from an environment variable.

This PR adds support to reading the config base path from the environment variable WAYBAR_CONFIG_DIR. If it is set, but no configuration is found there, it falls back to the previous mechanism of using the default paths, without erroring.

[leophys]

Hi @Alexays! I am sorry for nudging you. Do you have the chance to review this PR?

Thanks again :)

[Alexays]

Sorry for the delay, lgtm :)

[leophys]

Nice, thanks!