feature request: support posting comment to GitHub discussion

[ktravers]

In addition to posting comments to issues and pull requests, it would also be helpful to be able to post to GitHub discussions. For example, my team uses discussions for 1) collecting feedback on features and 2) logging decisions, two scenarios where being able to easily capture related Slack threads in a comment would be a very useful time saver.

Current behavior:

gh slack -i https://github.com/monalisa/smile/discussions/1 https://monalisa.slack.com/archives/channelIdentifier/threadIdentifier
not a repository or issue URL: "https://github.com/monalisa/smile/discussions/1"

Requested behavior:

gh slack -i https://github.com/monalisa/smile/discussions/1 https://monalisa.slack.com/archives/channelIdentifier/threadIdentifier
https://github.com/github/monalisa/smile/discussions/1#discussioncomment-123