Fix incorrect typings

[Skillz4Killz]

This PR fixes the createChannel typings.

1. The current typings make it impossible for anyone to create a channel as it expects a string and Typescript throws errors because in reality it requires a number. Not being able to pass a number is preventing me from making channels. HIGH PRIORITY PLEASE Can we consider hotpatching this into master as its only typings changes but its a massive issue. 🙏
   

2. Although it is labelled as an optional at the moment, the type is required making it incorrect as well. This should also be fixed now to make it properly optional.
   

3. reason is a parameter and was not present in the typings. It is now.

4. Added some overloads to the typings for createChannel for it to be much cleaner for TS devs. Now depending on the type that is provided, it will return the proper Channel Type as well.

• Providing no type or providing the 0 type will return TextChannel
  
• Providing 2 will return VoiceChannel
  
• Providing 4 will return the CategoryChannel
  
• Providing any other number will return unknown. Let me know if you would prefer to keep this as <AnyGuildChannel>

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This PR also fixes the following:

• channelMentions typings and JSDocs. ChannelMentions can never be undefined.
• category.channels can never be undefined it will always be a collection. Also changed categor.channels from returning a Collection<AnyGuildChannel> => Collection<TextChannel | VoiceChannel>