Bear 2.1 R6 extruder derivation

Here is new versione derived from prusa R6 but with bltouch. It suit bear 2.1 linear rails version.

Bl-touch y alignment is X=26.9 Y=0, this has been changed from original Vertigo235 idea to minimize issue reported here below:

https://github.com/AnHardt/Marlin/issues/84

and with animation here:

https://www.youtube.com/watch?v=rPU4ZAPJTNc

Original carriage should be modified as well but I use linear rail bear and mine can't be used on standard printers.

There are some missing parts because they are not changed from original Prusa/Bear