Expose public setter for typingAttributes; update protocol and AppKit/UIKit

https://github.com/krzyzanowskim/STTextView/discussions/14#discussioncomment-16449049

- STTextViewProtocol: typingAttributes is now get/set
- STTextView (AppKit/UIKit): change from public internal(set) to public var
- Enables clients to set typing attributes directly (API-breaking)