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.
it would be cleaner to just hae parseAndCheckModifiers return an array of modifier tokens.  You can then create the declaration nodes when necessary (not needing the .Unknown trick), and then assign the modifier tokens to it