Support "loading" attribute similar to Drupal image field formatters

Created MR 38 that adds the loading attribute support.

Static patch for composer builds.

Seems straightforward and I guess copied from core, but the default is eager, not lazy, so I think it should default to that to not change the behavior.

https://developer.mozilla.org/de/docs/Web/API/HTMLImageElement/loading

Changed default loading to eager. The issue needs review.