GraphQL parser swallows invalid escaped

[pcarrier]

The string generated by #2569 passes the graphql-js 15.0.0 parser just fine despite being invalid GraphQL. I would suggest being more conservative with invalid escape sequences even if it's a breaking change, this lets bad GraphQL loose.

[IvanGoncharov]

@pcarrier Thanks for reporting, answered by @hereisnaman and me in #2569