Enhancement: Image Compare Block: Missing responsive images

### Impacted plugin

Jetpack

### What

Image Compare Block: Missing responsive images

### How

The image comparison block outputs images without `srcset` attributes. This can be a performance problem when uploading large images.

Here's an example HTML markup that's produced by the block

```

```

For example, if I uploaded a 5000px wide image in this theme, it would always load that image, even though it only needs to be 650px wide.

Start by locating the Jetpack Image Compare block's rendering entry point and inspect how the two image URLs become the shown HTML. Confirm the change by checking that generated image markup includes responsive sources and that a large upload no longer always loads its full-size version.

Written by the indexing model from the issue text.