M701-M702 support

[charlespick]

Is your feature request related to a problem? Please describe.
Currently OctoDash supports 2 ways of loading filament from the filament screen during a print. M600 and normal movement commands. I don't want to use M600 because I don't have a screen connected to Marlin and normal movement commands don't work because they affect the E value in marlin meaning that the second that you resume printing, your filament unloads if you're using absolute positioning gcode.

Describe the solution you'd like
A 3rd option, configurable in the settings menu to use M701 and M702.

Additional context
Note I am not suggesting what was suggested in #1549 but rather just the same functionality as the existing non-M600 filament change behavior but using different commands.

[Wob76]

I would like to see this also.

I am running Octodash on the BIQU BX and as such I switch between Octodash and the printer LCD on the same screen, I tend to just stay on Octodash as if you switch when the printer is doing something it can have issues.

I was using the default Octodash inbuilt script for filament changes, but I find more often than not it will Jam on the unload (this doesn't happen during the M600 as it does a small purge\pause and then retract which seems to help shape the tip)

I tried switching to M600, this avoids the jamming, but has a few drawbacks, firstly it requires more X-Y motion as the M600 parks the print head during this process. Secondly it requires user prompts, this currently requires switching the LCD or using action prompts in octoprint, neither are ideal.

Implementing M701 and M702 would help over a M600 as it would reduce the unnecessary X-Y Motion and also allow the process to be completed with minimal action prompts, currently M701 still has a Purge More\Continue action prompt. You can also pass unload and load values to M701/M702 with L or U values, so they could also use the values from the settings if wanted, otherwise they will use the values stored in M603.

So I would suggest the interface can remain the same, but enabling this option would just replace the current E motion steps that are currently scripted with the M701 and M702 instead. I would further suggest that there be an option to enable each individually, in my example M702 (unload) offer advantages (it completes the tip forming steps) but M701 (load) has a drawback of requiring action prompts which octodash doesn't yet support.