Work around bug in graphql-php

The class `\GraphQL\Language\Parser` from the webonyx/graphql-php package throws an `Syntax Error: Unexpected Name "INTERFACE"` exception when the token `INTERFACE` comes last in the `DirectiveLocations` part of a directive declaration.
Adding `OBJECT` as the last item removes the error.

Author: Vinai

app/code/Magento/GraphQl/etc/schema.graphqls

@@ -37,7 +37,7 @@ directive @resolver(class: String="") on QUERY
     | INPUT_OBJECT
     | INPUT_FIELD_DEFINITION
 
-directive @typeResolver(class: String="") on INTERFACE
+directive @typeResolver(class: String="") on INTERFACE | OBJECT
 
 type Query {
 }