Replace serializers with toJSON and [Default] extended attribute

We just removed serializers from WebIDL and allowed editors to specify toJSON operations directly instead (WebIDL treated toJSON as a reserved identifier up until now).

To deal with common cases, we added a new [Default] extended attribute which triggers the default toJSON operation that behaves similarly to how serializers={attributes} or serializers={attributes, inherit} used to.

All serializer-related productions were removed from the WebIDL grammar. In particular the following non-terminals:

• Serializer
  • Serializer was also removed from the rhs expression of InterfaceMember.
• SerializerRest
• OperationRest
• SerializationPattern
• SerializationPatternMap
• SerializationPatternList

and the following terminals:

• serializer in ArgumentNameKeyword.

As usual, please feel free to reach out if anything needs clarification or if you have questions.

Thanks!