This change updates the NewsChannel classes so that the overwrite permission-related properties no longer throw an Exception when they are used.
These properties were not initially supported by News/Announcement channels when the feature was first released, but now they are.
* 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* remove (very outdated) todo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
