code: func parseTypeExtensionDefinition(parser *Parser) (ast.Node, error) {
  	start := parser.Token.Start
  	_, err := expectKeyWord(parser, lexer.EXTEND)
  	if err != nil {
  		return nil, err
  	}
  
  	definition, err := parseObjectTypeDefinition(parser)
  	if err != nil {
  		return nil, err
  	}
  	return ast.NewTypeExtensionDefinition(&ast.TypeExtensionDefinition{
  		Loc:        loc(parser, start),
  		Definition: definition.(*ast.ObjectDefinition),
  	}), nil
  }
docs: /**
   * TypeExtensionDefinition : extend ObjectTypeDefinition
   */