[webcodecs] Missing tasks in parallel steps in WebCodecs

[dontcallmedom-bot]

While crawling WebCodecs, the following algorithms fire an event, or resolve or reject a Promise, within a step that runs in parallel without first queuing a task:

• The ImageDecoder/isTypeSupported(type) algorithm resolves/rejects a promise directly in a step that runs in parallel

See Dealing with the event loop in the HTML specification for guidance on how to deal with algorithm sections that run in parallel.

_{Cc @dontcallmedom @tidoust}

_{This issue was detected and reported semi-automatically by Strudy based on data collected in webref.}