Consider adding virtual key code constants for A-Z and 0-9

[indianakernick]

Virtual key codes for A-Z and 0-9 are defined on MSDN but they aren't defined as constants in the Windows API headers. VK_A does not exist and so it doesn't exist in this crate. Could these constants be added? Perhaps this goes against what this crate is about. I would have opened a PR but I thought there would be a slim chance of it being merged. I just thought I'd bring it up anyway.