Schema extensions don't get added to the schema

[wravery]

When I tried using extend schema I realized that it wasn't adding those operations to the resulting code.

The Generator::visitDefinition method is calling the same method to visit either a schema definition or a schema extension, and since I split those into different AST types to improve the error messages and reflect the fact that they're actually different grammar rules, it's not finding any of the children it expects in a schema extension.

[wravery]

Need to merge first