Add support for package and element icons and screenshots

For catalogs we'll want to display icons for packages and elements. I'm not sure what formats are best to use - we should probably look to PWA and extension manifest formats to see how they do it.

Start by comparing how PWA and browser extension manifest formats represent package and element icons and screenshots, then review the repository's custom-elements manifest format and catalog needs. Done means the supported formats and metadata shape are agreed and documented for both packages and elements.

Written by the indexing model from the issue text.