Allow creating NSFW voice/stage channel

[Soheab]

Summary

This PR makes it possible to create a voice or stage channel marked as NSFW by adding the nsfw kwarg to both create methods. Editing was already possible.

Relevant API docs: https://discord.com/developers/docs/resources/guild#create-guild-channel

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] 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, ...)