Add support for multi-resolution mobile/desktop images (srcset)

[gcushen]

Feature Request

Add srcset for multi-resolution mobile/desktop images whilst also migrating from lazysizes to native lazy loading.

~~Add data-srcset for lazy loading multi-res mobile/desktop images. Docs: https://afarkas.github.io/lazysizes/index.html~~

May increase Lighthouse score.

• [x] Hero
• [x] Images/Figures
• [ ] Gallery
• [ ] Views (list items)
• [ ] Featured image