Add `first` to Message class

[preeded]

Pull request summary

Add first to Message class.

I just added first attr to Message that using first-msg tag, and updated changelog.rst.

Maybe it's about this discussion.

I've tested to connecte some channel with a bot and chated something. Message.first worked well I think.

Checklist

• [x] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
  • [x] I have updated the changelog with a quick recap of my changes.
• [ ] This PR fixes an issue.
• [x] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

[preeded]

Thank you for the review! I reflected it. I'm sorry for my poor Python skill. Finally, I think this code is okay.

[preeded]

Thank you! I reflected it in the code, and it looks good!