Add `is_nsfw` check to channels

A new feature in Discord is warning users about NSFW channels. This can
be useful when wanting to determine if a command can be used in a
channel or not, depending on the command content.

To help with this, provide a utility function named `utils::is_nsfw`.
This function accepts a channel name, which determines if the channel
is NSFW.

This information is not provided with data from the server. It is
determined client-side based on a few rules.

The rules for a channel being NSFW are:

- must be a guild channel
- must be a text channel
- must be named `nsfw` or be prefixed with `nsfw-`

If any of these conditions are false, then the channel is not NSFW.

Additionally, provide four helper methods:

- `GuildChannel::is_nsfw`: follows rules
- `Group::is_nsfw`: always false
- `PrivateChannel::is_nsfw`: always false
- `Channel::is_nsfw`: depends on inner channel (one of 3 above)

This check is volatile, as Discord may change requirements at any time.
The check provided by the library should not be taken as being accurate
all the time.

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