loop_sdcard: Add loopable SD card file sections

To support continuous belt printing, add nestable repeat
loop support via an `[sdcard_loop]` module.

Supported G-Code:
  - SDCARD_LOOP_BEGIN COUNT=n  ; Loop for N times, or infinitely if N is 0
  - SDCARD_LOOP_END            ; End of loop
  - SDCARD_LOOP_DESIST         ; Complete all loops without iterating

Marlin M808 compatibility example in `config/sample-macros.cfg`:
  - M808 Ln        ; Loop for N times, or infinitely if N is 0
  - M808           ; End of loop
  - M808 K         ; Complete all loops without iterating

Added unit tests in test/klippy/sdcard_loop.test

See https://reprap.org/wiki/G-code#M808:_Set_or_Goto_Repeat_Marker

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>

test/klippy/sdcard_loop.cfg

@@ -0,0 +1,89 @@
+# Test config for sdcard_loop
+[virtual_sdcard]
+path: test/klippy/sdcard_loop
+
+[display_status]
+
+# Override to support unlimited belt size
+# (homing Z simply resets its virtual position to 0.0)
+[homing_override]
+axes: xyz
+set_position_x: 0
+set_position_y: 0
+set_position_z: 0
+gcode:
+  G92 X0 Y0 Z0
+
+
+[stepper_x]
+step_pin: ar54
+dir_pin: ar55
+enable_pin: !ar38
+step_distance: .0125
+endstop_pin: ^ar3
+position_endstop: 0
+position_max: 200
+homing_speed: 50
+
+[stepper_y]
+step_pin: ar60
+dir_pin: !ar61
+enable_pin: !ar56
+step_distance: .0125
+endstop_pin: ^ar14
+position_endstop: 0
+position_max: 200
+homing_speed: 50
+
+[stepper_z]
+step_pin: ar46
+dir_pin: ar48
+enable_pin: !ar62
+step_distance: .0025
+endstop_pin: ^ar18
+position_endstop: 0.5
+position_max: 200000000
+
+[extruder]
+step_pin: ar26
+dir_pin: ar28
+enable_pin: !ar24
+step_distance: .004242
+nozzle_diameter: 0.500
+filament_diameter: 3.500
+heater_pin: ar10
+sensor_type: EPCOS 100K B57560G104F
+sensor_pin: analog13
+control: pid
+pid_Kp: 22.2
+pid_Ki: 1.08
+pid_Kd: 114
+min_temp: 0
+max_temp: 210
+
+[heater_bed]
+heater_pin: ar8
+sensor_type: EPCOS 100K B57560G104F
+sensor_pin: analog14
+control: watermark
+min_temp: 0
+max_temp: 110
+
+[mcu]
+serial: /dev/ttyACM0
+pin_map: arduino
+
+[printer]
+kinematics: cartesian
+max_velocity: 300
+max_accel: 3000
+max_z_velocity: 5
+max_z_accel: 100
+
+[sdcard_loop]
+
+[gcode_macro M808]
+gcode:
+    {% if params.K is not defined and params.L is defined %}SDCARD_LOOP_BEGIN COUNT={params.L|int}{% endif %}
+    {% if params.K is not defined and params.L is not defined %}SDCARD_LOOP_END{% endif %}
+    {% if params.K is defined and params.L is not defined %}SDCARD_LOOP_DESIST{% endif %}