Support reading environment variables in the config file

[nickjj]

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.

[nickjj]

I was able to get around this with a symlink but it could still be a nice to have.

Upon messing with this I noticed that relative paths such as include=./config.local do not work either, it results in mako throwing an error saying the path isn't found. This is potentially a separate issue but worth calling out.