Descriptions on executable documents

[fotoetienne]

Enhance GraphQL parser and printer to support descriptions for operations, variables, and fragments

This commit is based on #3402 , rebased onto graphql-js 16 and with added tests

Implements graphql/graphql-spec#1170

[benjie]

Looks good! We should have a test to ensure that a description on an anonymous query results in a sensible parse error if we don't already have one, plus the comment I made before. Other than that, I think this is good to go!

"""Invalid"""
{ __typename }