Refactors tokenizer to emit a Token double-linked-list

This refactors the tokenizer, simplifying both tokenizer and parser code slightly (fewer utility functions) and returns tokens as part of the AST's `loc` information. Tokens also have gained a few new properties: `line`/`column` for the 1-indexed position the token begins and `prev`/`next` to navigate the dll of Tokens. Finally, this introduces three new Token types: `<SOF>` for the empty token at beginning of the file stream, `,` for ignored comma tokens, and `Comment` for ignored comment tokens. These tokens are *included* in the Token list allowing for easier navigation of the parsed document in GraphQL tools and as a basis for parsing comment descriptions in the schema language.

Author: leebyron

src/index.js

@@ -121,6 +121,7 @@ export {
   visitInParallel,
   visitWithTypeInfo,
   Kind,
+  TokenKind,
   BREAK,
 } from './language';