asn2wrs: Added support for #.SYNTAX_NEW

This allows syntaxes to be registered as "new" dissectors.

Change-Id: Ibb76688d9d3cbf47222610956aa627fa374bcbce
Reviewed-on: https://code.wireshark.org/review/4524
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>