Issue:

This doesn't work with the latest FontAwesome v5 icons because a lot of the icons don't start with fa fa-xxx. Some start with fab or fas.

Here are a few examples:

fab fa-accessible-icon
fas fa-align-justify

File: widgets\Menu.php
Line: 31

Currently is:
public static $iconClassPrefix = 'fa fa-';

Should be:
public $iconClassPrefix = 'fa fa-';

Then the corresponding fix on line 84 to this:

: '<i class="' . $this->iconClassPrefix . $item['icon'] . '"></i> ',

Reason:

I am using the latest FontAwesome icons from https://fontawesome.com/icons?d=gallery&m=free loaded via their CDN. So I am not using your built in, out-dated version of FontAwesome.

Implementing this simple fix, would allow us to continue using your plugin along with the latest FontAwesome.

Yes, we would have to set the $iconClassPrefix to '', an empty string, and manually pass the prefixes ourself. Like so:

<?= Menu::widget([
    'options' => ['class' => 'sidebar-menu'],
    'iconClassPrefix' => '',
    'items' => $items,
]) ?>

This would only take a couple minutes to update, until you get the rest of this package updated.