Add ForumChannel.members property

[LostLuma]

Summary

Allows more easily checking who can see a particular forum channel.
This can be especially useful when using thread.parent.members, since previously only TextChannel had this property.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• 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, ...)