Backlight - Carve out a better location for private driver functionality (#8329)

* rename backlight_soft to match rules.mk

* rename backlight_soft to match rules.mk - update common_features

* Carve out a better location for private driver backlight functionality

quantum/backlight/backlight_driver_common.h

@@ -0,0 +1,7 @@
+#pragma once
+
+void backlight_pins_init(void);
+void backlight_pins_on(void);
+void backlight_pins_off(void);
+
+void breathing_task(void);