This PR moves NotificationCenter into swift-foundation.

• NotificationCenter is now defined in swift-foundation, with some methods kept in swift-corelibs-foundation via extension. This has the result of removing the ability to override these methods, e.g. addObserver(forName:object:queue:block) cannot be open in an extension.
• NotificationCenter no longer conforms to NSObject in swift-corelibs-foundation due to the lack of NSObject in swift-foundation, where NotificationCenter is now defined, and the inability to add that inheritance retroactively in swift-corelibs-foundation.
• Core logic for NotificationCenter was moved to swift-foundation and is accessed via @_spi(SwiftCorelibsFoundation) public methods. This enables methods like addObserver(forName:object:queue:block) to remain in swift-corelibs-foundation.