`<Menu>` doesn't close items dropdown on `Escape`

[achambers]

We should make the <Menu> dropdown close when Escape is hit. I suspect it should go here.

ember-headlessui/addon/components/menu/items.js

Lines 6 to 20 in 57f3da1

	onKeydown(event) {
	switch (event.key) {
	// Ref: https://www.w3.org/TR/wai-aria-practices-1.2/#keyboard-interaction-12
	case 'Enter':
	if (this.args.activeItem) {
	this.args.activeItem.element.click();
	}
	this.args.closeMenu();
	break;
	case 'ArrowDown':
	return this.args.goToNextItem();
	case 'ArrowUp':
	return this.args.goToPreviousItem();
	}
	}

[alexlafroscia]

I'm making some other improvements to Menu right now, including implementing the item search functionality. I'll take care of this one!