Add newly_created field to ThreadChannel

[Shea4]

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Description

Adds a new method isNewlyCreated to ThreadChannel so you can differenciate when a ThreadChannel is created or if the self member joined the thread

[MinnDevelopment]

Isn't this field only applicable to CHANNEL_CREATE events? I don't think it makes much sense to put that on the actual channel entity, when its primarily used to distinguish events.

[Shea4]

It is yes. Would a seperate event for THREAD_CREATE events be better or a method on the ChannelCreateEvent which throws if it's not for a thread, or any other suggestions you may have

[DV8FromTheWorld]

I'm gonna need to read up more on how we'd use this, but this definitely should not be on the entity itself.

[DV8FromTheWorld]

Superseded by https://github.com/DV8FromTheWorld/JDA/pull/2254