feat: added newly_created bool to threads

[Jaskowicz1]

This PR adds a newly_created bool to threads. As on_thread_create is also fired when a thread is added to an existing private thread (for example, adding a thread to a forum channel), there was no way to distinguish the actual creation event, Discord provides a newly_created bool so we now allow users to see that.

I avoided using the flags here in-case Discord add more flags (as dpp::channel_flags is close to its limit) to channels/threads.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[braindigitalis]

threads aren't cached, so bool is fine here

[Jaskowicz1]

threads aren't cached, so bool is fine here

Ah good good!