Add an MK3S+/MMU2 variant

[WizardStan]

This adds a new variant for those with MK3S+ and an MMU2 to remove the "different printer" warning.

[github-actions]

All values in bytes. Δ Delta to base

Target	ΔFlash	ΔSRAM	Used Flash	Used SRAM	Free Flash	Free SRAM
MK3S_MULTILANG	0	0	247692	5654	6260	2538
MK3_MULTILANG	0	0	247062	5663	6890	2529

[3d-gussner]

@WizardStan Thanks for the PR but we will not merge this.

• Additional hex files will confuse some users which firmware they should choose and flash. Fewer and fewer support issue are based on flashing the MK3 firmware on a MK3S printer, but sometimes it happens. We all started and some point and made mistakes.
• The PrusaSlicer profiles MK3S MMU3 and MK3S MMU2S only differ in the name
  • Please use only the MK3S MMU3 PrusaSlicer profile to easily avoid this issue.
  • Also "old" MMU2S gcode files and be modified by replacing M862.3 P"MK3SMMU2S" with M862.3 P"MK3SMMU3"

I don't want to force anyone to buy the MMU2S to MMU3 upgrade kit, but the upgrades to the plastic parts, hardware and electronics really improved the reliability of the MMU.