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.
this parameter should be named `existingNode` (I'll rename it in the iteration). When we parse declaration we pre-create node so it will be used to store information about parsed modifiers, see `parseDeclaration`. Here in statement we don't have modifiers -> no need to create node upfront