Bug 1578173 part 1. Add support for constructor operations in the parser. r=edgar

The grammar changes parallel those in whatwg/webidl#700

We don't prevent having both a constructor operation and [Constructor] or
[ChromeConstructor], because those extended attributes are about to get
removed, once they are no longer used in our IDL.

Differential Revision: https://phabricator.services.mozilla.com/D45387

UltraBlame original commit: 937448e0b594ed4108b7e376305dcdd877325354

Author: marco-c