API - Implement 'glfwGetKeyName'

[bryphe]

The glfwGetKeyName API would be useful to have: https://www.glfw.org/docs/latest/group__input.html#ga237a182e5ec0b21ce64543f3b5e7e2be

I'm not sure what the JS equivalent would be, so for now, we could just implement this on the native side (and a stub with a warning in the JS code).

The one change I would consider with this API is returning the integer equivalent of the returned string (for example, GLFW_KEY_A should return 65: https://github.com/bryphe/reason-glfw/blob/d128062ef93afd54f0d42cf128c54e79d46be231/src/glfw_key.re#L126).

So our signature for glfwGetKeyName could look like (key: int, scancode: int) =>Glfw_Key.t

This is primarily for performance consideration - if we return the string to the OCaml side, we'd have to copy the string into GC-managed memory, whereas passing the integer does not require copying.