feat: Add spoiler channel flag and functions

[ToothyDev]

Summary

This adds ChannelFlags.is_spoiler_channel (following https://docs.discord.food/resources/channel#channel-flags) as well as a .is_spoiler() function to all applicable channel types (all except category).

Channel editing with this flag is also functional, although stage channels seem to be special in that they have the options of nsfw and spoiler in the client, but it seems to have no effect. Thus I skipped adding it there for the time being, as nsfw is also not documented as a valid edit option for stage channels as of now.

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.
• I have updated the changelog to include these changes.
• AI Usage has been disclosed.
  • If AI has been used, I understand fully what the code does
• I have read the Contributing Guidelines

[pycord-app]

Thanks for opening this pull request!
Please make sure you have read the Contributing Guidelines and Code of Conduct.

This pull request can be checked-out with:

git fetch origin pull/3252/head:pr-3252
git checkout pr-3252

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3252/head

[Lulalaby]

take a shower <3

[Lulalaby]

took a shower apparently

[Lulalaby]

Can someone request changes reminding me to add editing & creating channels with that flag
and whatever else i may have forgotten

[Soheab]

Needs a way for bots to create this kind of channel

This message was sponsored by toothyfernsan on discord.

[ToothyDev]

I tested it with all but MediaChannels since I don't have access to those anywhere, but I'd still like a second pair of testing eyes on this

[ToothyDev]

Also, creating a channel with this turned on isn't possible unfortunately, gotta create + edit

[ToothyDev]

This feature is now documented (https://docs.discord.com/developers/resources/channel#channel-object-channel-flags) and thus ready to merge
The docs do state "Can only be set if channel’s nsfw is false", but I'm not sure whether I should add a mutually-exclusive check to both attributes, or just document this specific behavior

[ToothyDev]

The API will autoconvert spoiler channels to NSFW channels, but not NSFW channels to spoiler channels. I assume this is because they want to prevent people from accidentally disabling NSFW on channels.
I chose to make it convert both ways instead, since it's easier logically for the user, and results in MUCH less stupid logic checking code.. I sat an hour on trying to reflect the API's way before giving up.
The only time the library will reject user input now is if both options are passed as True