* add News channel type

* remove (very outdated) todo

* add [Socket/Rest]NewsChannel types

* update TextChannel properties to include a Type optional parameter with validation

as of writing, this feature is still only available to verified guilds, which makes it impossible for testing.

* actually create the news channels when given the type

* throw NotSupportedException in News channel

throw a NotSupportedException whenever trying to use SlowModeInterval or anything related to overwrite permissions

* make RestNewsChannel throw NotSupportedException also

* remove the (untested) ability to change channel types