[WIP] Generate additional mime types on upload

[adamsilverstein]

Summary

• Add a new filter image_editor_output_formats that returns the output formats and their order. The first type is used as the default output format
• The default image_editor_output_format filter applies for the first mime type, which also creates a full sized image if not the dame mime type as the uploaded image.
• New sub sizes and a full sized image are created for each mime type in the returned array

Usage

By default, this code will generate WebP and jpeg sub sizes for every uploaded image, with WebP as the default.

• Adding add_filter( 'image_editor_output_formats', '__return_false' ); would ensure only the uploaded mime type would be use for sub sizes (current WordPress behavior).
• Adding add_filter( 'image_editor_output_formats', function() { return array( 'image/jpeg', 'image/webp' ); } ); would ensure jpeg and WebP were generated, with jpeg stored in sizes and used as the default output format
• Adding add_filter( 'image_editor_output_formats', function() { return array( 'image/webp' ); } ); would ensure only WebP sub sizes were generated (current behavior of the webp-uploads module).

Fixes #

Relevant technical choices

Checklist

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[felixarntz]

@adamsilverstein We need to be careful here to not introduce this logic entirely on the server-side, to avoid timeouts. This relates to the enhancements from https://core.trac.wordpress.org/ticket/40439 - the additional sizes in the other format have to somehow be kicked off from the client-side, so that the server can take care of them through individual requests.

[felixarntz]

@adamsilverstein Do we still need this PR to be open? I'm thinking it's probably somewhat of a duplicate of #147 which is being worked on now.

[adamsilverstein]

Closing, this was an experimental PR.