fix: add missing values to const and all in channel.py

[AstreaTSS]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change
• CI change
• Other: [Replace with a description]

Description

Just a couple of fixes for some missing channel types in some places.

Changes

• Added GuildForumPost to __all__.
• Added GuildPublicThread, GuildForumPost, and GuildPrivateThreadtoTYPE_GUILD_CHANNEL`.
• Added GuildForumPost to TYPE_THREAD_CHANNEL.
• Added GuildForumPost to TYPE_MESSAGEABLE_CHANNEL.

Test Scenario(s)

...not much to test here.

Checklist

• I've formatted my code with Black
• I've added docstrings to everything I've touched
• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x
• I've tested my changes
• I've added tests for my code - if applicable
• I've updated the documentation - if applicable