Add servo mode support for spindle PWM control

- Implemented USE_SPINDLE_SERVO_MODE configuration option
- Changed Timer2 prescaler from 1/8 to 1/1024 for ~50Hz servo frequency
- Added configurable servo pulse width range (SPINDLE_SERVO_MIN_PULSE/MAX_PULSE)
- M3 S0-S1000 now controls servo position (0-180 degrees)
- M5 positions servo to maximum position (pen up) instead of disabling PWM
- Default configuration supports pen plotter with reversed pulse values (MIN=31, MAX=16)
- Only supports ATmega328p (Arduino Uno) processors

Based on: https://www.buildlog.net/blog/2017/08/using-grbls-spindle-pwm-to-control-a-servo/

debug.md

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+this is my curennt grbl state
+
+when homing the first limit switch encoutnered always retrun from homing. 
+it can be either the x or y
+how to fix
+```
+*** Connecting to jserialcomm://ttyUSB0:115200
+*** Fetching device status
+>>> ?
+<Alarm,MPos:0.000,0.000,0.000,WPos:1.002,-209.000,0.000>
+*** Fetching device version
+*** Fetching device settings
+>>> $$
+$0 = 10    (step pulse, usec)
+$1 = 25    (step idle delay, msec)
+$2 = 0    (step port invert mask:00000000)
+$3 = 2    (dir port invert mask:00000010)
+$4 = 0    (step enable invert, bool)
+$5 = 0    (limit pins invert, bool)
+$6 = 0    (probe pin invert, bool)
+$10 = 3    (status report mask:00000011)
+$11 = 0.010    (junction deviation, mm)
+$12 = 0.002    (arc tolerance, mm)
+$13 = 0    (report inches, bool)
+$20 = 1    (soft limits, bool)
+$21 = 0    (hard limits, bool)
+$22 = 1    (homing cycle, bool)
+$23 = 6    (homing dir invert mask:00000110)
+$24 = 25.000    (homing feed, mm/min)
+$25 = 500.000    (homing seek, mm/min)
+$26 = 250    (homing debounce, msec)
+$27 = 1.000    (homing pull-off, mm)
+$100 = 113.821    (x, step/mm)
+$101 = 100.000    (y, step/mm)
+$102 = 100.000    (z, step/mm)
+$110 = 8000.000    (x max rate, mm/min)
+$111 = 8000.000    (y max rate, mm/min)
+$112 = 8000.000    (z max rate, mm/min)
+$120 = 1200.000    (x accel, mm/sec^2)
+$121 = 1200.000    (y accel, mm/sec^2)
+$122 = 3800.000    (z accel, mm/sec^2)
+$130 = 310.000    (x max travel, mm)
+$131 = 210.000    (y max travel, mm)
+$132 = 100.000    (z max travel, mm)
+ok
+*** Fetching device state
+*** Connected to GRBL 0.9i
+>>> $G
+[G0 G54 G17 G21 G90 G94 M0 M5 M9 T0 F0. S0.]
+ok
+```