stepper: Make step pulse duration customizable at run-time

Remove the STEP_DELAY Kconfig option and replace it with a per-stepper
step_pulse_duration printer.cfg config option.

The AVR code will continue to have optimized code to step and "unstep"
in the same function (which is automatically activated when the step
delay is 40 ticks or less).  This change removes the Kconfig option
for single function step/unstep on 32bit processors.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Reference.md

@@ -153,6 +153,11 @@ microsteps:
 #   gear_ratio is specified then rotation_distance specifies the
 #   distance the axis travels for one full rotation of the final gear.
 #   The default is to not use a gear ratio.
+#step_pulse_duration:
+#   The minimum time between the step pulse signal edge and the
+#   following "unstep" signal edge. This is also used to set the
+#   minimum time between a step pulse and a direction change signal.
+#   The default is 0.000002 (which is 2us).
 endstop_pin:
 #   Endstop switch detection pin. If this endstop pin is on a
 #   different mcu than the stepper motor then it enables "multi-mcu