Make step to add pressure in the nozzle configuable

Add user configurable variable for the extrude distance in the step adding pressure in the nozzle in the prime line macro

macros/helpers/prime_line.cfg

@@ -7,7 +7,8 @@ gcode:
     {% set prime_line_height = params.LINE_HEIGHT|default(printer["gcode_macro _USER_VARIABLES"].prime_line_height)|default(0.6)|float %}
     {% set prime_line_adaptive = params.ADAPTIVE_MODE|default(1)|int %}
     {% set prime_line_margin = params.LINE_MARGIN|default(printer["gcode_macro _USER_VARIABLES"].prime_line_margin)|default(5.0)|float %} # Used only in adaptive mode
-
+    {% set prime_line_charge_length = params.CHARGE_LENGTH|default(printer["gcode_macro _USER_VARIABLES"].prime_line_charge_length)|default(18)|float %}
+
     # If the SIZE parameter is defined and not a dummy placeholder, we use it to do the adaptive bed mesh logic
     {% set coordinatesFound = false %}
     {% if params.SIZE is defined and params.SIZE != "0_0_0_0" %}
@@ -110,7 +111,7 @@ gcode:
 
     # Add pressure in the nozzle
     G92 E0
-    G1 E18 F300
+    G1 E{prime_line_charge_length} F300
 
     # Prime line
     G92 E0

user_templates/variables.cfg

@@ -59,6 +59,7 @@ variable_prime_line_purge_distance: 30 # length of filament to purge (in mm)
 variable_prime_line_flowrate: 10 # mm3/s used for the prime line
 variable_prime_line_height: 0.6 # mm, used for actual cross section computation
 variable_prime_line_margin: 5  # distance of purge line from fl_size rectangle
+variable_prime_line_charge_length: 18 # length of filament to pressurize the nozzle before starting the move
 
 ## Park position used when pause, end_print, etc...
 variable_park_position_xy: -1, -1