feat(GuildChannel): createInvite target options

[almostSouji]

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

Invite accepts a target as documented in:

• Fix create invite target_user_id field and add public_flags to example user objects discord/discord-api-docs#1708
• Document application invites and update docs for stream invites discord/discord-api-docs#2690 (also fixes regression of the former)

Target can be a user (has to be streaming in the VC the invite targets) or an embedded application (has to play VC activity in the VC the invite targets).

In order to facilitate this the following changes have been made:

• IntegrationApplication: add missing properties to
• GuildChannel: .createInvite(...) add and handle option keys targetType, targetUser and targetApplication
• Introduce ApplicationResolvable typedef
• QoL: Invite#targetUser should be "of this invite", "for" is used for creation
• QoL: specify ClientApplication#cover to be the hash
• ⚠️ QoL: rename TargetUser to TargetType (more in line with actual function)
• ⚠️ QoL: rename Application#coverImage to #coverURL for consistency with other CDN getters

TODO:

• What does hook (boolean) indicate in application data?
• What does verify_key (string) indicate in application data?
• Document and handle verify_key

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)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[almostSouji]

Later extended and corrected in discord/discord-api-docs#2690 updated OP

[kyranet]

For consistency with many other methods (e.g. Guild#iconURL, Guild#splashURL, User#avatarURL, etc):

stale