Add native image lazy loading support

[Cherry]

As of Chrome 76, native lazy loading has come to the web!

Setting images in blog posts to loading="lazy" (or user-configurable) could provide massive performance benefits, especially on blog posts with lots of images. The engineering team over at the BBC did a write-up about this and showed page load times reduced by 50%.

https://web.dev/native-lazy-loading

Wordpress is also looking to do this by default very soon as per https://core.trac.wordpress.org/ticket/44427.

Whilst this is only currently supported in Chrome, Safari has expressed interest, and Chrome does account for over 50% of the web.

This is a completely non-destructive and backwards-compatible change, so implementing it now will only mean further benefits in the future as other browser vendors implement the spec.

PS. I know that feature requests generally aren't tracked here, but I was advised to create this issue as per https://forum.ghost.org/t/add-native-image-lazy-loading-support/8799/2

[daviddarnes]

@Cherry My bad, features are kept on the forum as feature requests on the repo are typically closed. We can track interest on the forums over here https://forum.ghost.org/t/add-native-image-lazy-loading-support/8799. Sorry for the confusion 🙂

[angristan]

Looks like it's been added as part of v4! 074ce4e