Add Vector image auto_sizes support

[xf-]

Read images sizes from vector files to be more consistent with "photo" images.

[mahagr]

I made some minor improvements to improve error handling.

[xf-]

@mahagr i used that code nested and adjusted it more or less to your code style in grav and removed some PHP8 stuff like $xml?-> and next line checks if it is an element :)

I thought about using try catch and also xml is requirement in composer. Normally every web server should use it and avoid inconsistent behavior. i noticed mbstring is suggested in composer and index.php basically stops without it.

THX for the quick merge

[mahagr]

I didn't have any PHP8 code, $attr = $xml ? $xml->attributes() : null; works in PHP5 and is required as loading the file may fail (non-XML file).

[mahagr]

Fixed mb extension issue, too.