Add a variable distance for end_print extruder pull back.

[rongith]

I like to have the end_print pull back distance a variable.
Why: I have different toolheads with different hot zone lengths and I like the filament pulled in the cold zone according to the toolhead.

[wight554]

@rongith thanks for raising PR
you might want to remove hardcode of 18mm here as well
https://github.com/Frix-x/klippain/blob/main/macros/helpers/prime_line.cfg#L113

In my issue I suggested retracting same amount as you pre-extrude in the following line
Might be good idea to create another option for this case as well

[rongith]

@wight554 , I may do an other PR for that. Perhaps a better practice.
Here you want to keep a relation between the "pressure in the nozzle" before prime in PRIMELINE and the previous pull in END_PRINT?

[wight554]

I honestly feel like at start you should push amount you retracted in the end, so purging is more predictable. But we can configure both separately