feat(backlight): added percentexp as format option

Humans don't perceive light linearly and so an exponential curve is more
appropriate to gauge the perceived brightness of the screen.

src/modules/backlight.cpp

@@ -179,9 +179,17 @@ auto waybar::modules::Backlight::update() -> void {
 
     if (best->get_powered()) {
       event_box_.show();
+
       const uint8_t percent =
           best->get_max() == 0 ? 100 : round(best->get_actual() * 100.0f / best->get_max());
+
+      const uint8_t percent_exp =
+          best->get_max() == 0
+              ? 100
+              : roundf(powf((float)best->get_actual() / best->get_max(), 1.0f / 2.718f) * 100);
+
       label_.set_markup(fmt::format(format_, fmt::arg("percent", std::to_string(percent)),
+                                    fmt::arg("percentexp", std::to_string(percent_exp)),
                                     fmt::arg("icon", getIcon(percent))));
       getState(percent);
     } else {