htmlimageelement: Reject decode promises on the current request mutation (#37828)

Follow the HTML specification and reject the pending image decode
promises
on any current request mutation.
https://html.spec.whatwg.org/multipage/#updating-the-image-data (step
18)
https://html.spec.whatwg.org/multipage/#dom-img-decode (step 3)

Fulfill and reject image decode promises by queueing
a global tasks on the DOM manipulation task source.

Testing: Improvements in the following tests
- html/semantics/embedded-content/the-img-element/decode/*

Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>

components/script/dom/bindings/error.rs

@@ -97,6 +97,7 @@ pub(crate) fn throw_dom_exception(
         Error::NotReadable => DOMErrorName::NotReadableError,
         Error::Operation => DOMErrorName::OperationError,
         Error::NotAllowed => DOMErrorName::NotAllowedError,
+        Error::Encoding => DOMErrorName::EncodingError,
         Error::Type(message) => unsafe {
             assert!(!JS_IsExceptionPending(*cx));
             throw_type_error(*cx, &message);