Add onwheel

[vhiairrassary]

Scala-js-dom currently supports onmousewheel but as written on MDN:

Do not use this wheel event. This interface is non-standard and deprecated. It was used in non-Gecko browsers only. Instead use the standard wheel event.

As mentioned above it is not supported on Firefox. This PR adds onwheel.

You can test with the following code:

import org.scalajs.dom
import org.scalajs.dom.html.Div

object Main {
  def main(args: Array[String]): Unit = {
    val onwheelChild = dom.document.createElement("div").asInstanceOf[Div]
    onwheelChild.textContent = "onwheelChild"
    onwheelChild.onwheel = event => {
      println("onwheel: " + event)
    }
    dom.document.body.appendChild(onwheelChild)
    
    val onmousewheelChild = dom.document.createElement("div").asInstanceOf[Div]
    onmousewheelChild.textContent = "onmousewheelChild"
    onmousewheelChild.onmousewheel = event => {
      println("onmousewheel: " + event)
    }
    dom.document.body.appendChild(onmousewheelChild)
  }
}

⚠️ MouseWheelEvent is different than the standard WheelEvent, and Scala-js-dom currently uses WheelEvent for the deprecated onmousewheel. I didn't want to change the Scala-js-dom code as it would break the compatibility for the current users. Note: on my computer, Chrome returns a WheelEvent for both events. Here is a screenshot showing the output of the above snippet:

Also I didn't add documentation. Feel free to ask me to do it if required

[japgolly]

Thanks for the PR and your patience @vhiairrassary!