feat(PartialGroupDMChannel): Add recipients & fix name

[Jiralite]

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

• Added recipients (array of objects consisting of only usernames)
• Name is null when a group direct message channel does not yet have a name (initially created)

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)

[ImRodry]

How do you obtain this object? Also wouldn’t the name be the preset name consisting of the names of the recipients separated by commas?

[Jiralite]

How do you obtain this object?

1. Create a group direct message channel
2. Create an invite to it
3. Client#fetchInvite

Also wouldn’t the name be the preset name consisting of the names of the recipients separated by commas?

Whilst the client shows the names of the people in the group direct message or "Unnamed" if you are alone, Discord sends null.