Add missing definition for matrix falcon

[ValdisThomann]

VIA firmware already exists for this board, but no v3 json. This PR fixes that.

Description

QMK Pull Request

Already in main
https://github.com/qmk/qmk_firmware/tree/master/keyboards/matrix/falcon

VIA Keymap Pull Request

Already in main
https://github.com/the-via/qmk_userspace_via/tree/main/keyboards/matrix/falcon/keymaps/via

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