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,
};

[mrnoisytiger]

It would be great to allow more than 16 user-defined keycodes. I have opened a PR on VIA to mimic the same: the-via/app#44

It appears to be purely a UI issue. VIA recognizes the USER17-USER** keycodes in the UI but it is grayed out and not selectable.