Provide a way for a reviewer to make private notes, tasks, or bookmarks in a pull request

When I'm reviewing a pull request, there are times when I would like to add a private note, task, or bookmark (or something else) related to a piece of code. This is often useful if I want to come back to a piece of code after reviewing other parts first, perhaps because it may refer to another related class that I want to review first.

Existing alternatives:

• Use a comment - visible to everyone and would just be noise at that point as it may not actually be an issue
• Make a note outside the pull request - difficult to link to a specific section of code
• Create a comment, but leave it in draft - hard to find later and has the potential to be saved for everyone