Recognize the "required" keyword for dictionary members.

Created by: beverloo

The WebIDL specification defines a concept of required dictionary members, which have the "required" keyword prepending their type.

http://heycam.github.io/webidl/#required-dictionary-member

I ran into this when converting the Notification specification to use Bikeshed, which in turn uses the widlparser.

I'm not sure how to judge the test output. There are no (new) syntax error messages, and the complexity goes up by one. Is that sufficient?