Enable comments and discussions on gists for collaboration

[saismrutiranjan18]

Add a commenting system to allow discussions on gists, similar to GitHub issues.

Features:

• Comment on specific lines (like GitHub code reviews)
• General comments at bottom
• Markdown support in comments
• @mentions for notifying other users
• Edit and delete own comments
• Reactions/emoji responses
• Comment threading/replies
• Subscribe to comment notifications

[cassidoo]

This is a cool idea, and I might add some kind of comments component where the source of truth for said comments is the actual comment section in GitHub Gists. I won't close this one yet because I want to research it first, but currently this is not planned. That might change and I'll be sure to credit this issue if so.