[Feature request] implement a RETRO_DEVICE for mahjong

Following up https://github.com/libretro/FBNeo/issues/1057 where a user tried to play mahjong games on switch.

While the language barrier made things awkward, i believe what he meant by "mahjong mask" is not

but something akin to the virtual keyboards and button overlays that are common on mobile devices.

I think a good solution would be the implementation of a RETRO_DEVICE_MAHJONG.
The benefits include :

• a standardized layout, like the retropad
• awareness in the frontend, so that it can handle them however it wants (remapping to keyboard/gamepad, virtual mahjong controller, ...)

Currently, core developpers are basically being forced to map mahjong games to RETRO_DEVICE_KEYBOARD, which is probably far from ideal for mobile device users.

FWIW "mahjong controllers" also exist on consoles :

Start by reading the linked libretro/FBNeo issue and this proposal's discussion to understand the required mahjong controls and frontend expectations. Done means agreeing on the RETRO_DEVICE_MAHJONG interface, its standardized layout, and how frontends can handle it; the issue does not name implementation files or tests.