WebIDL is at https://html.spec.whatwg.org/multipage/embedded-content.html#htmlimageelement

Spec is at https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-decode

How to do it:

1. Add the decode member to our WebIDL
2. Implement the required method on our dom struct

Doing 2 will require integrating the decode concept with the ImageContext and the ImageCache

Look at the async decoding and handling of the data happening in the image cache, and find a way to hook it up to a promise in script.

See also https://html.spec.whatwg.org/multipage/images.html#img-decoding-process