v0.0-269-g4ecb59b: Parse nonblocking event trigger operator '->>'. LRM 15.5.1

Also factored out a kRepeatControl grammar rule for reuse.

'delay_or_event_control' now matches LRM description.

Renamed event trigger constructs to distinguish between blocking/nonblocking.

Also handle new operator in formatter token_annotator.
Adds new and missing tests for '->' and '->>'.

fixes #227

PiperOrigin-RevId: 299368975