Safe way to convert an u8 to a Vk

[kzagar]

I suggest adding a safe way to convert an u8 value to a Vk, by being able to verify if a given u8 value corresponds to a virtual key, i.e., if there is an enum in Vk that maps to that u8 value.

Without such mechanism, there is no robust way to convert an u8 value (as received e.g., from a Windows hook) to a Vk for further use.