Consider generating fallback JPEG/PNG when a WebP/AVIF is uploaded originally

Feature Description

This was raised by @webd-uk in a support forum topic: Open Graph protocol does not support webp.

I thought what was being requested was something that was already implemented (cf. https://github.com/WordPress/performance/issues/2028) where outputting modern image formats is skipped during wp_head for the sake of social providers that don't support them (e.g. Open Graph).

However, the issue is rather if you uploads a WebP or AVIF from the beginning (as WordPress now supports). In this case, the plugin won't need to do anything since a modern image format is the original format. In this case, there's no JPEG/PNG to fall back to render in wp_head.

Perhaps this plugin should support site owners who are already using WebP/AVIF images by generating the JPEG/PNG both for use in wp_head and also for PICTURE element fallbacks.

Start by tracing the existing handling for modern image formats in wp_head and the PICTURE element fallback behavior. Determine how an originally uploaded WebP or AVIF could produce JPEG/PNG fallbacks for both uses, then verify the behavior with the relevant image and markup tests.