Add parsing context

[Schottkyc137]

Adds a ParsingContext to all parsing related functions instead of manually passing TokenStreams and potentially DiagnosticHandlers to the individual functions.

This is helpful for future expansions e.g., for more lax parsing, to have the DiagnosticHandler ready or to pass a VHDLStandard to the individual functions without having to drill the argument all the way down to functions down in the stack.