feat: Support for text in stage

[NeloBlivion]

Summary

Adds support for Text channels in Stage as per discord/discord-api-docs#5928
Closes #1934

• Now inherits abc.Messageable
• Updated some documentation here and there
• Moved nsfw slot to VocalGuildChannel

This PR is more or less a mirror of #1231 for StageChannel, however the implementation is somewhat unsatisfying; I think It'd be more ideal for VoiceChannel and StageChannel to inherit from _TextChannel and then move the thread-related functions & attributes from _TextChannel to TextChannel and ForumChannel, but to keep in scope I went with the basic route of just mimicking the previous changes with StageChannel. If the suggested alternative is considered a better solution then I'll go ahead with that refactor instead.

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.

[codecov]

Codecov Report

Merging #1936 (60ff3f1) into master (ba129e6) will decrease coverage by 0.02%.
The diff coverage is 26.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1936      +/-   ##
==========================================
- Coverage   33.33%   33.32%   -0.02%     
==========================================
  Files          97       97              
  Lines       18867    18901      +34     
==========================================
+ Hits         6290     6298       +8     
- Misses      12577    12603      +26     

Flag	Coverage Δ
macos-latest-3.10	33.30% <26.82%> (-0.02%)	⬇️
macos-latest-3.11	33.30% <26.82%> (-0.02%)	⬇️
macos-latest-3.8	33.31% <26.82%> (-0.02%)	⬇️
macos-latest-3.9	33.31% <26.82%> (-0.02%)	⬇️
ubuntu-latest-3.10	33.30% <26.82%> (-0.02%)	⬇️
ubuntu-latest-3.11	33.30% <26.82%> (-0.02%)	⬇️
ubuntu-latest-3.8	33.31% <26.82%> (-0.02%)	⬇️
ubuntu-latest-3.9	33.31% <26.82%> (-0.02%)	⬇️
windows-latest-3.10	33.30% <26.82%> (-0.02%)	⬇️
windows-latest-3.11	33.30% <26.82%> (-0.02%)	⬇️
windows-latest-3.8	33.31% <26.82%> (-0.02%)	⬇️
windows-latest-3.9	33.31% <26.82%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
discord/abc.py	22.33% <ø> (ø)
discord/message.py	24.95% <ø> (ø)
discord/state.py	14.70% <0.00%> (ø)
discord/channel.py	34.11% <27.50%> (-0.56%)	⬇️

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[BobDotCom]

I think It'd be more ideal for VoiceChannel and StageChannel to inherit from _TextChannel and then move the thread-related functions & attributes from _TextChannel to TextChannel and ForumChannel

What if we created a new class for thread related stuff, so it looks something like this:

_TextChannel
├── VoiceChannel
├── StageChannel
└── _ThreadsCapable
    ├── TextChannel
    └── ForumChannel

Could you make an issue for this? I think it's a good idea, but I also agree that it's out of the scope of this PR.

[BobDotCom]

Please add a changelog entry

done