feat: stage channels

[amishshah]

Please describe the changes this PR makes and why it should be merged:

Adds support for stage channels, and fixes voice states that use stage channels. Tracks discord/discord-api-docs#2751.

Also adds the rtc_region property to voice channels and stage channels.

I believe that this PR meets the current draft for the feature given by Discord. I think the following need to be considered before deeming this feature stable:

• Has API Documentation for Stage Channels. discord/discord-api-docs#2751 been merged?
• Has REQUEST_TO_SPEAK been removed or changed?

  "Allows for requesting to speak in stage channels. (This permission is under active development and may be changed or removed.)

  • The permission is still in flux but the documentation has been published for the feature.
• Has the logic for determining StageChannel#joinable been clarified?
  • See API Documentation for Stage Channels. discord/discord-api-docs#2751 (comment)

  ...yes, this bypass works right now, but this is more of an implementation detail and may change in the future.

  • Based on this, I implemented StageChannel#joinable to ignore this permission, since it seems hacky.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

[advaith1]

API docs PR: discord/discord-api-docs#2751

[ThaTiemsz]

Could add the new video_quality_mode channel property for voice and stage channels too (not for stage; discord/discord-api-docs#2754 (comment)):

Property is not present in the channel object unless you manually change the setting. Then it's one of two values:

enum VideoQualityMode {
    AUTO = 1,
    FULL = 2
}

{
  id: '209719953706844162',
  type: 2,
  name: 'General',
  position: 0,
  parent_id: '355904666480934913',
  bitrate: 63691,
  user_limit: 0,
  rtc_region: null,
  video_quality_mode: 2,
  guild_id: '209719953706844161',
  permission_overwrites: [Array],
  nsfw: false
}

[amishshah]

@NotSugden @BannerBomb I opted to create a BaseGuildVoiceChannel class that both StageChannel and VoiceChannel now extend from. StageChannel and VoiceChannel have quite a few more differences than TextChannel and NewsChannel, so proper inheritance was easier in this case