Allow comment UI to be triggered via a clickable icon in the gutter

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Following up on a conversation with @isidorn [here](https://github.com/TeamCodeStream/codestream/issues/908). We've love to be able to use the comment API, though we have some concerns about the current mechanism for initialing a comment. For our use, having a clickable icon in the editor gutter to kick off that comment would be the ideal UX (see attached image below) <img width="234" alt="Screen Shot 2022-04-11 at 3 49 06 PM" src="https://user-images.githubusercontent.com/834948/162818178-0a4d6411-9c55-4067-95de-5dc90b32501e.png">