feat(sitemap): add image sitemap

[andrmoel]

Allow image sitemap following the official schema: https://developers.google.com/search/docs/advanced/sitemaps/image-sitemaps

Usage:

    public function generateItems(): void
    {
        ...

        $this->items[] = [
            'loc' => 'https://www.foo.bar',
            'lastMod' => 123456789,
            'image' => [
                'loc' => 'https://www.foo.bar/image.jpg',
            ],
        ];
        
        ...
    }