[Export2Slicer] Use step format in export

[dlozinski]

Use step format since Prusa slicer supports step files now.

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[galou]

Why do you think that Prusa's mesher is better than FreeCAD's? I'd guess that it's better to use FreeCAD's mesher because you avoid an intermediate step, in both sense of the word.

[dlozinski]

Why do you think that Prusa's mesher is better

I didn’t say that.

From my experience step worked better - resulted in less errors during import into slicer.

[galou]

Please keep the default format as it was and only add STEP format as an option. Mesh formats are more widely supported.