Add User keycodes

[knoopx]

Add user keycodes, as defined by VIA:

enum user_keycodes {
    USER00 = 0x5F80,
    USER01,
    USER02,
    USER03,
    USER04,
    USER05,
    USER06,
    USER07,
    USER08,
    USER09,
    USER10,
    USER11,
    USER12,
    USER13,
    USER14,
    USER15,
};