Larger support for discussions

[wd60622]

I would like to see larger support for discussions with Octo nvim after #643 is merged. This issue aims to break down the work needed into smaller tasks.

• Build out a discussion buffer
• Add support for interacting with the discussions
  • comment on a discussion
  • react to a discussion
  • mark a discussion as resolved
• create a new discussion

This is not a complete list of tasks and feel free to breakdown more granular tasks as needed.