Needed for #30822

Spec: https://drafts.csswg.org/resize-observer/#resize-observer-interface

One would have to add a new WebIDL(see components/script/dom/webidl), and expose the interface on Window.

And then implement it in a new components/script/dom/resizeobserver.rs file. The WebIDL interface methods will be generated automatically and can be imported as use crate::dom::bindings::codegen::Bindings::<interface name>

For the callback part, components/script/dom/eventtarget.rs is probably a good place to look for inspiration.