Sniff for getimagesize()

## What problem would the enhancement address for VIP?

`getimagesize()` holds up the PHP process downloading the entire image to just read its file size.

## Describe the solution you'd like

Most images in WPs Media Library have width and height attributes already (like from `wp_get_attachment_metadata()`) & that existing data should be used instead.

If the image is external and not in WordPress' media library, perhapes:
* a VIP helper function could be created [to download only first few bytes of image](https://www.php.net/manual/en/function.getimagesize.php#88793)
* don't use this function, or do image sizing w/ JS or CSS instead

## What code should be reported as a violation?

```php
$size = getimagesize( $image_url );
list($width, $height) = getimagesize( $image_url );
//etc.
```

Start with the getimagesize() violation examples and the proposed wp_get_attachment_metadata() path, then review how the ruleset identifies PHP function calls. Clarify the expected handling for Media Library images versus external URLs before defining completion and adding coverage for both cases.

Written by the indexing model from the issue text.