LS: include tokens in tree during parsing 

Currently parser drops lots of token information on the floor which is fine for batch compilation. However language service still needs this data so to reconstruct it we now re-scan the source text. This PR contains proposal for implementation strategy to augment the parser with a few extensibility points so LS can plug in into the parsing process and embed token information into the tree during parsing.
Would it be possible to just pass back the token itself and map from the token to the property name? I'm not really comfortable with how the property names are hard-coded into the parser, and I'd be willing to help to make this happen.