fix(GuildChannel): clone its PermissionOverwriteManager too

[SpaceEEC]

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

This PR fixes #6082 by cloning GuildChannel#permissionOverwrites when cloning the channel itself.
This PR also addresses the issue that a passed iterable would cause the then created PermissionOverwrites to be without a channel as the channel property would be initialized after the parent's constructor added them.
(This might be better solved with another private method to be called from the deriving classes, e.g. this._init(iterable); or similar)

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