Feature Request: support contextmenu

It would be nice if we could have a helper for `contextMenu` like we have for `click`. I am manually triggering it in my tests, as I could not find a way to trigger it with ember-test-helpers.

I am doing this:

```js
const element = document.querySelector(
'[data-test-palette-row="First Palette"]'
).parentElement;

var contextMenuEvent = new MouseEvent('contextmenu', {
bubbles: true,
cancelable: false,
view: window,
button: 2,
buttons: 0,
clientX: element.getBoundingClientRect().x,
clientY: element.getBoundingClientRect().y
});

element.dispatchEvent(contextMenuEvent);
```

Start by locating the existing `click` helper and its tests, then compare their event-dispatch behavior with the issue's manual `MouseEvent('contextmenu')` example. Done means a `contextMenu` helper can trigger the same event without manual DOM selection and is covered by tests.

Written by the indexing model from the issue text.