Enable end user opt-in to generate all sizes in fallback format

[b1ink0]

Summary

Fixes #1513

Relevant technical choices

This PR adds a checkbox users can check right below the "generate fallback images" checkbox (and it would only be enabled when "generate fallback images" checkbox is checked).

When users enable this feature it is notate in the Generator tag so we can later discover how many sites opt into generating all image sizes.

Settings Page Screenshots

Sample outputs for the below custom sizes

add_image_size( 'custom_size_1', 400, 400, true );
add_image_size( 'custom_size_2', 800, 800, true );
add_image_size( 'custom_size_3', 1200, 1200, true );

• When the new option is off

• When the new option is on

[github-actions[bot]]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

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

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.