[Implement] isVoid<T>()

[jtenner]

For combined use with ReturnType.

if (isFunction<T>()) {
  if (isVoid<ReturnType<T>>())  {
    call_indirect(func, ...props)
  } else {
    let value: ReturnType<T> = call_indirect(func, ...props);
    // do something with value
  }
}

[jtenner]

closed in favor of #739