Spindle speed overrides behavior tweak. New experimental laser dynamic power mode.

- Spindle speed overrides now update immediately if they are changed
while in a HOLD state. Previously, they would update after exiting the
HOLD, which isn’t correct.

- New experimental dynamic laser power mode that adjusts laser power
based on current machine speed. Enabled by uncommenting
LASER_CONSTANT_POWER_PER_RATE in config.h

  - It assumes the programmed rate is the intended power/rate for the
motion.
  - Feed rate overrides (FRO) do not effect the power/rate. Meaning
that spindle PWM will automatically lower with lower FRO and increase
with higher FRO to keep it the same.
  - Spindle speed overrides (SSO) will directly increase and decrease
the power/rate. So 150% SSO will increase the PWM output by 150% for
the same speed.
  - The combination of FRO and SSO behaviors should allow for subtle
and highly flexible tuning of how the laser cutter is operating in
real-time and during the job.

- Re-factored planner block rapid rate handling for the dynamic laser
power feature. Should have had no effect on how Grbl operates.

grbl/config.h

@@ -569,7 +569,12 @@
 #define PARKING_PULLOUT_RATE 100.0 // Pull-out/plunge slow feed rate in mm/min.
 #define PARKING_PULLOUT_INCREMENT 5.0 // Spindle pull-out and plunge distance in mm. Incremental distance.
                                       // Must be positive value or equal to zero.
-                                      
+        
+// Experimental feature that adjusts laser power (PWM output) based on current running speed of the
+// machine. When laser mode is enabled, Grbl will turn off the spindle PWM pin whenever it is not 
+// moving, which may be confusing to some users to why their laser is not on. This behavior may change
+// in future iterations of this feature, where it will turn on to the minimum rpm value when active.
+// #define LASER_CONSTANT_POWER_PER_RATE
 
 /* ---------------------------------------------------------------------------------------
    OEM Single File Configuration Option