feat: implement (Sync)WebhookMessage.thread

[Def-Try]

Summary

Related to issue #2298
Fixes webhook messages assuming that their _thread_id can not be None (spoiler; it can)

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[Dorukyum]

Your changes might fix the issue but I think the best way to approach this is to create a property called WebhookMessage.thread_id instead. That would get rid of the duplicate code. What do you think?

[Lulalaby]

I agree with doru

[Def-Try]

Wait, i read comment wrong...

[Def-Try]

so, something like that? never worked on big projects (and typing to be honest), so my style may be weird

[Def-Try]

i do not know why docs/readthedocs.org:pycord check keeps failing
everything else on the other hand appears to be ok

[Lulalaby]

https://readthedocs.org/projects/pycord/builds/23121803/ look here

[Def-Try]

https://readthedocs.org/projects/pycord/builds/23121803/ look here

all i see here is an AttributeError: None with no information about what caused it (in pycord code i mean) and a lot of warnings

[Dorukyum]

I went ahead and replaced your solution with adding WebhookMessage.thread and SyncWebhookMessage.thread properties. Not sure if this is necessary or if this is the best solution, but it's the cleanest solution that came to my mind.

Requires testing.

[Dorukyum]

After looking with a clear mind into the issue I realized none of these changes were necessary. Will open a new pr with just 2 lines changed. Thanks nevertheless.