Native lazy loading

Problem/Motivation

Browsers now supports native lazy loading on iframes: https://web.dev/iframe-lazy-loading/

Steps to reproduce

Insert an iframe on a node and add display:none; on it.
The iframe is loaded even if not displayed.

Proposed resolution

The iframe module should add a loading="lazy" attribute on iframes.

It could be optional because there are some use cases where you don't want lazy loading (iframe that are at the top of the page for example).