Add <img loading=lazy> "lazy loading images"

Follow-up: #5236.

Tests: 
* web-platform-tests/wpt#14914
* web-platform-tests/wpt#17985
* web-platform-tests/wpt#18087
* web-platform-tests/wpt#20634
* web-platform-tests/wpt#20747
* web-platform-tests/wpt#20683
* web-platform-tests/wpt#20774
* web-platform-tests/wpt#20680
* Some as part of https://bugzilla.mozilla.org/show_bug.cgi?id=1542784

Closes #2806. Closes #5250.

Co-authored-by: Dominic Farolino <domfarolino@gmail.com>
Co-authored-by: Anne van Kesteren <annevk@annevk.nl>