Make ReadableStreamReadResult<T> type more strict

[nwtgck]

Hi, developers. Thank you very much for your project!

I'm very new to contribute to this repo.
This PR makes ReadableStreamReadResult<T> more strict.

Please verify that:

• There is an associated issue in the Backlog milestone (required)
• Code is up-to-date with the master branch
• You've successfully run gulp runtests locally
• There are new or updated unit tests validating the change

References

• https://developer.mozilla.org/en-US/docs/Web/API/ReadableStreamDefaultReader/read#Return_value
• https://developer.mozilla.org/en-US/docs/Web/API/ReadableStreamBYOBReader/read#Return_value

[typescript-bot]

It looks like you've sent a pull request to update some generated declaration files related to the DOM. These files aren't meant to be edited by hand, as they are synchronized with files in the TSJS-lib-generator repository. You can read more here. For house-keeping purposes, this pull request will be closed.