feat: add support for getting a threads message

[LordOfPolls]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

Allows a user to easily access the parent message for a thread, or the child thread of a message.

Changes

• Determine parent message on thread create
• Add parent_messsage property to threads
• Add thread property to message

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code

[xCraith]

Tested message.thread and it works perfectly as requested!