Extending JsxDOM.domProps

I would make the callback return promise<unit> since it’s likely the most common signature instead of 'a. The real issue comes from the input of this callback function, you can’t make its type depend on a dependency, the cleanest solution would likely to add a type formData to runtime/Dom.res, we’ll see how we swap this with the experimental WebAPI bindings in the future. Apart from that, it looks ok!