Add is_nsfw to voice channels

[Middledot]

Summary

Now that you can send messages in voice channels, you can also set it to be age-restricted.

Checklist

• 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
• 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, ...)

[codecov]

Codecov Report

Merging #1317 (aaad573) into master (fb3065d) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #1317   +/-   ##
=======================================
  Coverage   33.42%   33.42%           
=======================================
  Files          93       93           
  Lines       17610    17615    +5     
=======================================
+ Hits         5886     5888    +2     
- Misses      11724    11727    +3     

Flag	Coverage Δ
pytest	33.42% <50.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
discord/channel.py	37.10% <50.00%> (+0.02%)	⬆️

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