[Motivation](https://github.com/NixOS/nixpkgs/issues/257817#issuecomment-1741705042):
- Having all the XKB options in the same attribute set clarifies their
relation better than using a common option name prefix ("xkb").
- `services.xserver.layout` is an XKB option, but this is not obvious
from its name. Putting it with the other XKB options clarifies this.
Co-authored-by: Michele Guerini Rocco <rnhmjoj@users.noreply.github.com>
