creality.ini: introduce slow abl variant start_gcode
creality.ini: introduce slow abl variant start_gcode

printers with a large bed are probably more prone to heat induced
bed warping, there we'll split up our abl start_gcode in fast
and slow variants, where printers with a smaller bed like
the Ender-3 will still use the fast variant, printers with
a large bed like the CR-10 will use the slow variant which
heats up the bed before starting the abl procedure.

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