Added iframe render element

A render element for iframe has been added.

Previously

$build['video'] = [
    '#type' => 'html_tag',
    '#tag' => 'iframe',
    '#attributes' => [
        'src' => 'https://example.com/video.webm',
        'allow' => 'fullscreen',
        'width' => 420,
        'height' => 315,
        'loading' => 'eager',
        'class' => ['foo', 'bar'],
    ],
];

Now

$build['video'] = [
    '#type' => 'iframe',
    '#src' => 'https://example.com/video.webm',
    '#allow' => 'fullscreen',
    '#width' => 420,
    '#height' => 315,
    '#loading' => 'eager',
    '#attributes' => [
        'class' => ['foo', 'bar'],
    ],
];