File::setModel(): Switch from class_uses() to duck test

[lukasbestle]

The following lines cause a regression when a model class is extended in a plugin (because class_uses() does not check parents):

kirby/src/Filesystem/File.php

Lines 447 to 449 in cfdba1b

	if ($model !== null && in_array(IsFile::class, class_uses($model)) !== true) {
	throw new InvalidArgumentException('The model object must use the "Kirby\Filesystem\IsFile" trait');
	}

Instead we want to check for the existence of the root() and url() methods.

See mullema/k3-image-clip#25 for the original discussion.

[bastianallgeier]

✅