Add context key for when comments are visible

I was trying to get escape collapsing all comments (https://github.com/microsoft/vscode/issues/131419) but this would need an extra context key. Here are the current context keys:  I think ideally we would have `commentsAreVisibleInViewport` or `commentsAreVisible`, the latter isn't quite as good since it would hide comments which are currently off screen and might seem like nothing happened.