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.
Actually, I would think that the real perf gain would be when using it through the LS, since it would run over time and get the chance to be inlined. I might be wrong, but either way it's still probably a good idea.