Add placeholder variables that tell which tool is going to be used for a print job

[matyay]

Version

2.3.0+linux-x64

Operating system type + version

Ubuntu 20.04

3D printer brand / version + firmware version (if known)

Custom built, using Klipper

Behavior

This is a feature request.

In a multi-extruder setup currently there is no GCODE placeholder variable that would tell if a particular exturder/tool is used by a print job. It would be useful to have such variable(s) - with them a start GCODE macro could be crafted that would eg. enable heater and purge only the extruder(s) that are needed. This would allow to use a single printer profile of a multi-extruder machine and use it for printing with any of the extruders available. Currently there is a need for separate profile when eg. only the second extruder is to be used due to the need of custom preheat and purging GCODE.

[rtyr]

Duplicate of #5094. Closing.