feat(commands): generalize @is_nsfw() check for potential future channel types

[shiftinv]

Summary

Manually maintaining this list isn't great, but alternatively something like callable(getattr(ch, "is_nsfw", None)) and ch.is_nsfw() # type: ignore feels even worse.
New channel types don't get added all that often, after all.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running uv run nox -s lint
  • I have type-checked the code by running uv run nox -s pyright
• 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, ...)

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📚 disnake | 🛠️ Build #34511003 | 📁 Comparing 59ea97c against latest (a71a371)

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[Enegg]

Hmm, couldn't we just have a .is_nsfw() on each channel type?
Basically make it a part of the "Channel" interface

[shiftinv]

The remaining channels, i.e. categories and dm/group channels, don't really have a concept of nsfw (or spoiler content in #1577, for that matter), and unlike is_spoiler or is_obfuscated in other PRs I wouldn't be sure about reasonable defaults here

[shiftinv]

Realizing that this is perhaps a bit silly, as ultimately neither prefix commands nor any interactions can occur in the context of these channels directly.
iac, as long as the GuildMessageable type alias remains correct, a simple isinstance(ch, (abc.GuildChannel, Thread)) actually does it just fine.

[Enegg]

Rare instance of deleting lines of code :>