Add boardsource/sessanta

[waffle87]

Description

Adds v3 definition of Boardsouce Sessanta keyboard

QMK Pull Request

qmk/qmk_firmware:23823

Checklist

• The VIA support for this keyboard is MERGED in QMK 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

[Cipulot]

VIA keymap moved after last QMK BC here: https://github.com/the-via/qmk_userspace_via/tree/main/keyboards/boardsource/sessanta/keymaps/via