fix: sizes attribute on sourceset

[tijmenbruggeman]

When the original image contains sizes, we need to copy these sizes over to the new sources. According to the spec (https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Responsive_images), the sizes attribute on the original image are also required on the to select the right image.

<img src="test.jpg" srcset="test-360.jpg 360w, test-720.jpg 720w, test.jpg 1200w" sizes="(max-width: 480px) 360px, (max-width: 1024px): 720px, 1200px)" />

should create:

<picture>
  <source 
    type="image/webp" 
    srcset="test-360.webp 360w, test-720.webp 720w, test-1200.webp 1200w" 
    sizes="(max-width: 480px) 360px, (max-width: 1024px) 720px, 1200px" 
  />
  <img 
    src="test.jpg" 
    srcset="test-360.jpg 360w, test-720.jpg 720w, test.jpg 1200w" 
    sizes="(max-width: 480px) 360px, (max-width: 1024px) 720px, 1200px" 
    alt="Description of image"
  />
</picture>