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

[yume-chan]

The TypeScript definition for webcodecs in DefinitelyTyped is outdated, so I'm trying to generate a new one from official IDL using a modified version of microsoft/TypeScript-DOM-lib-generator.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

webcodecs/index.src.html

Lines 1565 to 1568 in 46eaed1

	dictionary AudioDecoderSupport {
	boolean supported;
	AudioDecoderConfig config;
	};

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.

[chcunningham]

Sounds good. Will do