Describe the bug
When calling graphql.schema.idl.SchemaPrinter#schemaPrinter with a schema that has graphql.schema.GraphQLSchema#extensionDefinitions defined, I would expect it to print out the "extend schema" string. I would expect the graphql.schema.idl.SchemaPrinter#schemaPrinter to:

1. Check if options.isUseAstDefinitions() returns true.
2. Print "extend schema" for each out.print(AstPrinter.printAst(extension));.

Alternatively, is there another method to enable printing of the extensions?