Implement “internally create a new object implementing the interface”

[ExE-Boss]

Extracted from https://github.com/jsdom/webidl2js/pull/213 because of https://github.com/jsdom/webidl2js/pull/213#issuecomment-792345633:

The generated snapshots here seem overcomplicated, and it's not clear why you're modifying existing functionality (like new()).

Blocks:

• https://github.com/jsdom/webidl2js/pull/213