Add support for a retainer to scrape the probe off

[Cova]

The popular Ender3 Klack probe was inspired by Klicky and family - but because of the more limited range of motion of the simple bed-slinger, it requires the use of a small retainer that scrapes the probe off of the toolhead at a certain Z position.

This adds a small section to the Dock_Probe macro to optionally perform that extra movement just before the decoupling movement.

[Schlonky]

This would be a nice addition, as the Klack probe has started becoming quite popular

[oldcrazyeye]

This would be amazing to have klackender supported

[jlas1]

Let me check this in more detail