nixos/xss-lock: add module (#40619)

`xsslock` (which was originally packaged in 6cb1d1aaaf02a72329bedf9c6960e54fea6f5c6e)
is a simple screensaver which connects a given screen locker (e.g.
`i3lock`) with `logind`. Whenever `loginctl lock-sessions` is invoked
the locker will be used to lock the screen. This works with its power
management features (e.g. `lid switch`) as well, so the PC can be locked
automatically when the lid is closed.

The module can be used like this:

```
{
  services.xserver.enable = true;

  programs.xss-lock.enable = true;
  programs.xss-lock.lockerCommand = "i3lock";
}
```

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