Use AstSchemaBuilder from args rather the default one

[Axxiss]

On the current implementation definitions(document, builder) doesn't use the builder argument at all and instead always use AstSchemaBuilder.default. So this PR make use of the custom builder and pass to the AstSchemaMaterializer as an argument.

 // This implementation provides the default AstSchemaBuilder
  def definitions(document: ast.Document): Vector[Named] =
    definitions[Any](document, AstSchemaBuilder.default)

 // Here we accept a custom AstSchemaBuilder as an argument but we don't use it. We use the default one instead
  def definitions[Ctx](document: ast.Document, builder: AstSchemaBuilder[Ctx]): Vector[Named] =
    new AstSchemaMaterializer[Ctx](document, AstSchemaBuilder.default).definitions

[travisbrown]

@Axxiss This change looks reasonable to me. Would you be interested in migrating this PR to the new GitHub organisation for review?

[Axxiss]

Moving the PR to the new orga 👍