Enhancements to support Intellij plugin

[jaccomoc]

Modifications to the compiler are needed to support an Intellij plugin. In particular we need to support recovering parsing after an error and support the concept of a Marker. Semantic content that needs representation in the plugin needs to be able to be marked at the start and then completed via a Marker.done() call at the end of the section. Markers can be nested and can be rolledback or dropped if not needed and also support Marker.error() for reporting parsing errors.