Add support for Plexus75

[Drauthius]

Description

Add Plexus75

QMK Pull Request

Already merged in: qmk/qmk_firmware#10668

Checklist

• The QMK source code follows the guide here: https://caniusevia.com/docs/configuring_qmk
• The VIA definition follows the guide here: https://caniusevia.com/docs/layouts
• 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