Add boardsource/sessanta

Description

Adds v3 definition of Boardsouce Sessanta keyboard

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