Support reading environment variables in the config file

For loading multiple configs you can pass in `include=~/hello/world/dotfiles/.config/mako/config` which is very handy to keep 1 main config file in 1 spot but allow you to override specific colors for different themes. This works wonderfully as is.

However, it means hard coding `~/hello/world/dotfiles`. It would be nice if mako supported reading environment variables so you can do `include=$DOTFILES_PATH/.config/mako/config` instead. This avoids hard coding user specific paths for folks who have dotfiles that are public and are used by others.

Hyprland supports this style of reading environment variables with its `source` property btw.