Make kill return Unit instead of Boolean

The underlying implementation in `node/lib/internal/child_process.js` does
return a boolean value, but this value is devoid of any semantic content.

Shortened it does:

```
if childProcessHandle {
  res = child.kill(signal)
  if (res === 0) {
    child.killed = true;
    return true;
  else {
    throwError()
  }
}
else {
  return false;
}
```

which means the returned boolean depends on whether the subprocess still exists
and whether the signal was successfully sent (apparently the original author
thought that sending a signal will always kill the process).

The return value isn’t even mentioned in the documentation, so it must be a
historical artifact kept around for backward compatibility.

Author: Profpatsch

src/Node/ChildProcess.purs

@@ -83,7 +83,7 @@ type ChildProcessRec =
   , stderr :: Nullable (Readable ())
   , pid :: Pid
   , connected :: Boolean
-  , kill :: String -> Boolean
+  , kill :: String -> Unit
   , send :: forall r. Fn2 { | r} Handle Boolean
   , disconnect :: Effect Unit
   }
@@ -125,10 +125,15 @@ send msg handle (ChildProcess cp) = mkEffect \_ -> runFn2 cp.send msg handle
 disconnect :: ChildProcess -> Effect Unit
 disconnect = _.disconnect <<< runChildProcess
 
--- | Send a signal to a child process. It's an unfortunate historical decision
--- | that this function is called "kill", as sending a signal to a child
--- | process won't necessarily kill it.
-kill :: Signal -> ChildProcess -> Effect Boolean
+-- | Send a signal to a child process. In the same way as the
+-- | [unix kill(2) system call](https://linux.die.net/man/2/kill),
+-- | sending a signal to a child process won't necessarily kill it.
+-- |
+-- | The resulting effects of this function depend on the process
+-- | and the signal and can vary from system to system.
+-- | The child process might emit an "error" event if the signal
+-- | could not be delivered.
+kill :: Signal -> ChildProcess -> Effect Unit
 kill sig (ChildProcess cp) = mkEffect \_ -> cp.kill (Signal.toString sig)
 
 mkEffect :: forall a. (Unit -> a) -> Effect a