Definitely assigned fields in return value dictionaries should be marked as `required`

The TypeScript definition for webcodecs in DefinitelyTyped is outdated, so I'm trying to generate a new one from official IDL using a modified version of microsoft/TypeScript-DOM-lib-generator. It worked, but old tests didn't pass.

The reason is that the old version was generated from Chromium IDL, which differs from official IDL.

For example, AudioDecoderSupport dictionary in official IDL is

https://github.com/w3c/webcodecs/blob/46eaed12fa374cd530986f4ad34ab0723463b8d0/index.src.html#L1565-L1568

and Chromium version

dictionary AudioDecoderSupport {
  required boolean supported;
  required AudioDecoderConfig config;
};

Looking at related steps, these fields are definitely assigned, so the required keyword is correct.

Marking them as required more precisely reflects shape of the value, and TypeScript-DOM-lib-generator needs them to generate more correct type definitions.

Start with the linked official WebCodecs IDL in index.src.html and compare the AudioDecoderSupport definition with the Chromium IDL linked in the issue. Check the related algorithm steps and the old test results, then verify that definitely assigned dictionary fields are represented as required and that the existing tests pass.