Added itemprop to allowed attributes

[FabianKoestring]

Allows to set something like the following. Is needed for setting structured data markup.
Include Your Site Name in Search Results.

$this->headLink()->prependAlternate(
    [
        'rel' => 'canonical',
        'itemprop' => 'url',
        'href' => 'https://example.com/'
    ],
    'APPEND'
);

<link rel="canonical" href="https://example.com/" itemprop="url">

[gianarb]

👍

[Martin-P]

Nice addition. Perhaps you can add a unittest for this?