Add element capture for images and PDFs

[stevebauman]

I've added element() so you can capture a specific part of an HTML document, Blade view, or URL:

$source = Screenshot::url($url)->element('#invoice');

$source->png()->save($imagePath);
$source->pdf()->save($pdfPath);

Images crop to the first matching element's bounds, including padding and borders. PDFs move the selected element into the document body and print it using the configured paper, margins, and print styles. Stylesheets are retained, but styles that depend on removed parent elements may need adjustment.

Each output builder keeps the selector chosen when it was created. Image clipping and PDF preparation live in their respective capture classes.