Expose core classes for custom usage

[robhawkes]

Description

• The way I am using the CityJSON parser differs to the general implementation available within the core exports
• For example, CityJSONWorkerParser does a lot though I want more control and customisation over geometry generation
• Another example is to implement a parser into my own worker scripts (CityJSONWorkerParser tries to create its own worker)
• Or I may want to create an entirely different data management mechanism like streams, or worker pools
• Manually accessing ChunkParser has been helpful for me but it's not accessible via the root / index file

Expectation

• Expose all the useful classes from the parser in the root / index file (at least ChunkParser and GeometryParser)
• Core classes can be used to perform basic parsing so I can perform my own geometry processing or mesh / material application