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.

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