Add support for modern image formats like WebP/AVIF

As an experiment author, I want to be able to upload images to experiments that are high quality, have low file sizes, and support transparency.

By supporting modern image file formats such as WebP or AVIF in the visual editor, users would be able to benefit from the advancement in image compression techniques in the 25–30 years since JPEG and PNG were developed that result in images with smaller file sizes for the same image quality.

Lower file sizes and/or higher quality images relative to a certain file size can help reduce the performance impact web experimentation has on Core Web Vitals resulting in better end-user experience, lower web experimentation's impact on SEO, and reduced chances of decision-makers feeling as though they need to choose between running an experimentation program and site performance.