Add VIA support for shuguet/shu89 split keyboard (PCB + Case are open-source)

[shuguet]

Description

Adding a keyboard and it's associated keymap.
This is an 89 keys split keyboard, made up of a single (breakable) PCB + a case (can be either 3D printed or CNC made).
Both case and PCBs are done, only the CNC technical drawings are left TODO.

QMK Pull Request

qmk/qmk_firmware#24758

VIA Keymap Pull Request

the-via/qmk_userspace_via#46

Checklist

• The VIA support for this keyboard is MERGED in QMK master already (MANDATORY)
• VIA keymap is MERGED in VIA userspace master already (MANDATORY)
• The VIA definition follows the guide here: https://caniusevia.com/docs/layouts
• I have a V3 JSON version for this keyboard definition.(MANDATORY)
• I have formatted the JSON file to have consistent formatting with the rest of the repository.
• I have tested this keyboard definition using VIA's "Design" tab.
• I have tested this keyboard definition with firmware on a device.
• I have assigned alpha keys and modifier keys with the correct colors.
• The Vendor ID is not 0xFEED