Add missing attributes in AppCommandChannel

[owocado]

Summary

This PR adds following missing attributes to AppCommandChannel class, the underlying payload for which is partial channel object as mentioned in upstream docs:

• category_id
• topic
• position
• last_message_id
• slowmode_delay
• nsfw (and x.is_nsfw() method)
• _last_pin
• _flags attr & flags @​property
• x.is_news() method for consistency because it exists for TextChannel

Bikeshedding post: https://discord.com/channels/336642139381301249/1355473537577652234

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• 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, ...)

[Rapptz]

These are all missing the .. versionadded:: string.

[owocado]

ok added