Add WebP and AVIF support

[manusfreedom]

• Add support for WebP and AVIF support
• Use PHP IMAGETYPE_x constant
• Remove WebP support from NetPBM

[tenzap]

will this work for users running php < 7.1 (eg. php 5.6) ? ci3 still supports these PHP old versions.
IMAGETYPE_AVIF constant doesn't seem known by php 5 https://php-legacy-docs.zend.com/manual/php5/en/function.exif-imagetype

[manusfreedom]

This is not supported in PHP 5.x, so for them you need to add a defined('IMAGETYPE_AVIF') OR define('IMAGETYPE_AVIF', 19); in constant.php..