Support creating standalone public threads

[fenhl]

Discord's API has two endpoints for creating threads: one for creating a thread as a reply to a message and one for creating a thread that's not connected to any existing message. The former only supports creating public threads, but the latter supports both public and private threads. However, serenity did not allow creating public standalone threads. This PR fills that gap, and renames the methods for calling these endpoints to more accurately describe what they do.

When I brought up the missing functionality in the #serenity channel, @arqunis claimed that the “start thread without message” endpoint was only for private threads, pointing to this part of the documentation:

* type currently defaults to PRIVATE_THREAD in order to match the behavior when thread documentation was first published. In a future API version this will be changed to be a required field, with no default.

I believe that this is a misunderstanding of the docs. They say “defaults to PRIVATE_THREAD”, not “must be PRIVATE_THREAD”. I also tested calling the endpoint manually with type set to PUBLIC_THREAD and it works as expected.

[fenhl]

CI failure seems to be unrelated.

[mkrasnitski]

Just a few minor things. I think mentioning that threads linked to messages will always be public is something we should do, so the docs change to the create_thread_from_message methods should be reverted. It might also be worth changing the docs on CreateThread::kind to mention how the field is ignored for message threads (as well as how it defaults to private for standalone threads).