feat(backlight): added percentexp as format option

[Narice]

Humans don't perceive light linearly, so an exponential curve is more appropriate to gauge the perceived brightness of a screen.
This is fully tested and should work everywhere (Tested with Nix).
The new format would need to be documented on the wiki.
The icon markup is based on the normal percent markup for now but I can make an iconexp.
Also, I don't know the syntax for markup, so if it needs to be percent_exp and icon_exp (which is more readable), I can do that as well

[Narice]

I actually did what I said preemptively:
percentexp -> percent_exp
added icon_exp
Tested and icon_exp is actually quite nice to have

btw, I use brightnessctl to be able to change my brightness with a certain exponent:

brightnessctl -n --exponent=2.718 s +1%

[Alexays]

Just fixed the conflict, sorry for the delay, could you update the man and the Wiki? 🙏