Schema Coordinates
Implements https://github.com/graphql/graphql-spec/pull/794/
Adds:
* DOT punctuator in lexer
* Improvements to lexer errors around misuse of `.`
* Minor improvement to parser core which simplified this addition
* `SchemaCoordinate` node and `isSchemaCoodinate()` predicate
* Support in `print()` and `visit()`
* Added function `parseSchemaCoordinate()` since it is a parser entry point.
* Added function `resolveSchemaCoordinate()` and `resolveASTSchemaCoordinate()` which implement the semantics (name mirrored from `buildASTSchema`) as well as the return type `ResolvedSchemaElement`
