feat(Voice): implement support for @discordjs/voice

[amishshah]

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

This PR allows for creating adapters that can be used by @discordjs/voice. This PR also removes the existing voice codebase as it will no longer be maintained.

The only voice code that will remain in discord.js will be voice states, which aren't specifically part of the Discord Voice API.

What is @discordjs/voice?

@discordjs/voice is a stand-alone library that implements the Discord Voice API. It aims to offer better stability, performance, and scalability than the existing voice client within Discord.js v12.

Discord.js v13 will remove support for the Voice API, and this functionality can be replaced by installing @discordjs/voice (npm install @discordjs/voice) as an add-on.

Example usage:

const { joinVoiceChannel } = require('@discordjs/voice');

const connection = joinVoiceChannel({
  channelId: channel.id,
  guildId: channel.guild.id,
  adapterCreator: channel.guild.voiceAdapterCreator,
});

How do I update my code to use @discordjs/voice?

@discordjs/voice is nearing its release. If you want to get a head start on migrating your voice code to the library, you should take a look at this PR which shows how to create a music bot with @discordjs/voice and TypeScript - discordjs/voice#107

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)

[amishshah]

I'm converting this back to a draft as I want to fix discordjs/voice#78 before adapters are added directly to Discord.js.

[amishshah]

Converting to a draft, need to update the adapter as well as deprecate current Discord.js voice code.