Add support for AVIF image format

[BugBuster1701]

• first Contao / Imagine: Ticket
• check support in getimagesize / exif_imagetype: Constants
• As an alternative via Extension: strtolower(pathinfo($imageFilename, \PATHINFO_EXTENSION));

[BugBuster1701]

• Test an test picture: https://libre-software.net/avif-test/

• function_exists( 'exif_imagetype' ) ... in older PHP versions only available if exif extension is activated

[BugBuster1701]

• getimagesize:

IMG_AVIF (int)
Used as a return value by [imagetypes()](https://www.php.net/manual/en/function.imagetypes.php) 
Available as of PHP 8.1.0.

[BugBuster1701]

'avif' => \defined('IMAGETYPE_AVIF') ? IMAGETYPE_AVIF : 19,
aus https://github.com/contao/contao/blob/4.13/core-bundle/src/Resources/contao/library/Contao/File.php#L270