Issue 55: Preloading responsive images

[rpasillas]

Summary

• Add support for preloading responsive images #55
• Fixes PHP Fatal by requiring alleyinteractive/composer-wordpress-autoloader composer package.

Example:

am_preload(
	[
		'handle'        => 'preload-wolf',
		'src'           => 'wolf.jpg',
		'version'       => '1.0.0',
		'as'            => 'image'
		'mime_type'     => 'image/jpeg',
		'imagesrcset'   => 'wolf_400px.jpg 400w, wolf_800px.jpg 800w, wolf_1600px.jpg 1600w',
		'imagesizes'    => '50vw',
		'fetchpriority' => 'high',
	]
);

Testing Instructions

1. Call the am_preload() like the example above.
2. Ensure you see the <link> in the head and that it meets the criteria in this article: https://web.dev/articles/preload-responsive-images

Checklist

• I have tested these changes locally.
• I have updated documentation as needed.
• I have added relevant tests if applicable.