Automatically opt into 1536x1536 and 2048x2048 sizes when generating fallback images

[b1ink0]

Summary

Fixes #1512

Relevant technical choices

This PR automatically opt into 1536x1536 and 2048x2048 sizes when generating fallback images. This only happens when the "generate fallback images" is enabled.

The opt in is done by modifying the $_wp_additional_image_sizes global which is the array containing extra image sizes. The provide_additional_mime_types property is set to true for the 1536x1536 and 2048x2048 image sizes so that the selected modern image format of these sizes is also generated.

Before

For the demo img these images were generated ( fallback images is enabled )

After

For the demo img these images are now generated ( fallback images is enabled )

[github-actions]

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Co-authored-by: b1ink0 <b1ink0@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

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[felixarntz]

@b1ink0 This basically looks good to me, only two outstanding comments to address.