Improve Streams

[MattiasBuelens]

This PR improves the type definitions for Streams:

• Align ReadableStreamReadResult with the new IteratorResult definition from TypeScript 3.6. It is now equivalent to IteratorResult<T, T | undefined>, but since the DOM types cannot depend on es2015.iterable, I needed to add my own typedef with a union type. Fortunately, TypeScript will recognize these as equivalent thanks to its structural type system. 😄 Since this is a generic type alias, I have updated the emitter to support type parameters for typedefs.
• Add chunk types to TextEncoderStream and TextDecoderStream from the Encoding standard. Their readable and writable properties were defined as just ReadableStream and WritableStream, so they had the default chunk type any. Now, we use the precise chunk types from the spec.

I've also done some cleanup on how the Streams types are defined:

• Use [NoInterfaceObject] in (manually authored) WebIDL, instead of adding "no-interface-object" to the overrides.
• Remove unnecessary "override-exposed" where the exposure was already correctly defined in the WebIDL.