Alternative brightness control option using brightnessctl

[Yocracra]

I can only control my ASUS TUF laptop using backlightctl, so I edited volume_brightness.sh and the i3 config to include a backlightctl option. While I was there, I made volume/brightness step size an argument for volume_brightness.sh so it can be set right in the i3 config.

If someone could test that the original method is still functional on their device, that'd be great.

This is my first ever PR so feedback is very much welcome :)