fix(typewriter): struct cannot extend structs

[leonmk-aws]

Fixes #

Typewriter was not rendering extends statements for structs.
Having:

  new StructType(scope, {
    name: 'MyStruct',
    extends: [scope.type('AnotherStruct')],
  });

Would not render the extends statement.

Description of the changes

Added a getter for the extends on the struct type and rendering code for it.

Testing

Added unit tests

[github-actions]

@aws-cdk/aws-service-spec: No model change detected