Lazy loading attribute support

[gjb2048]

Ref: https://developer.mozilla.org/en-US/docs/Web/Performance/Lazy_loading#images_and_iframes

Add support for "loading='lazy'" :).

Also info on https://caniuse.com/?search=loading

[brendanheywood]

Thinking if would be better if we converted over to this method but then added support back in for safari via a polyfill:

https://github.com/mfranzke/loading-attribute-polyfill