Adjust category and guild _channels attributes to work with NoneType positions

[NeloBlivion]

Summary

Adapts guild/category _channels attributes (text_channels, voice_channels etc.) to work correctly with Optional positions.
Currently when grabbing a list of a guild's or a category's channels, the library attempts to sort them by position to output them in their UI order:

r.sort(key=lambda c: (c.position, c.id))

This worked fine when we just had TextChannel and VoiceChannel because position under abc.GuildChannel was required, but since 2.0 with new channel types and interaction payloads position has been made optional for all channel types; since position can now be None, this sort breaks with TypeError: '<' not supported between instances of 'NoneType' and 'int'.

The solution i've applied is to assume the channel's position is -1 if position = None, so it will order as much as possible; other solutions could be to try-except, completely give up on sorting, or to make -1 the default instead of None (achieving the same result, but perhaps with a knock-on effect elsewhere in the library).

I went with -1 since this was actually already present in Guild.by_category(), where it's assumed ID and position are -1 if it runs into a NoneType category. Alternatively, it could be a large number to assume the position is last instead of first.

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.

[Dorukyum]

Looks good for now