Add create_activity_invite shortcut method

[CodeWithSwastik]

Summary

Adds a discord.VoiceChannel.create_activity_invite shortcut method to aid creating party game invites like youtube together etc.

Rationale

await vc.create_activity_invite('youtube')

would be easier to use and readable compared to

await vc.create_invite(target_type=discord.InviteTarget.embedded_application, target_application_id=755600276941176913)

P.S. Documentation can be improved.

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, ...)

[Luc1412]

I think the term "activity" might be better than "party". Also, an enum could be more convenient instead of using a String.

[CodeWithSwastik]

We already have enums for activities which could be confusing. Though I like create_activity_invite over create_party_invite