Media: Enable lazy-loading of images by automatically adding the new loading="lazy" attribute to image tags on the front-end.
- Introduces `wp_lazy_loading_enabled()`, `wp_filter_content_tags()`, `wp_img_tag_add_loading_attr()`, and `wp_img_tag_add_srcset_and_sizes_attr()` functions.
- Introduces `wp_lazy_loading_enabled`, `wp_img_tag_add_loading_attr`, and `wp_img_tag_add_srcset_and_sizes_attr` filters.
Props flixos90, addyosmani, mor10, swissspidy, pierlo, westonruter, spacedmonkey, mikeschroder, jonoaldersonwp, peterwilsoncc, narwen, jeffpaul, OptimizingMatters, futtta, mukeshpanchal27, azaozz.
Fixes #44427.
git-svn-id: https://develop.svn.wordpress.org/trunk@47554 602fd350-edb4-49c9-b593-d223f7449a82
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