Rule Request: Explicit NotificationCenter usage.

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.
   We are making a Mac application and some of the older macOS versions still store targets without incrementing their weak reference count, so forgetting to remove the observer will result in a crash, that's why we made utility functions for NSObjects to store the observations (since they can also be block based, just removing self wouldn't be enough) and remove them from notification center when the object deallocates.
   According to this, iOS9 and macOS 10.11 are the minimum version where this is not needed anymore:
   https://developer.apple.com/documentation/foundation/notificationcenter/1413994-removeobserver

2. Provide several examples of what would and wouldn't trigger violations.
   Trigger:
   NotificationCenter.default.addObserver(self, selector:selector, name:.notificationName, object:nil
   NotificationCenter.default.addObserver(forName:.notificationName, object: nil, queue: nil, using: {})
   Not trigger:
   NotificationCenter.default.post(.notificationName)

3. Should the rule be configurable, if so what parameters should be configurable?
   no parameters

4. Should the rule be opt-in or enabled by default? Why?
   opt-in since not everyone has to support OS versions where this is a problem.

No implementation file or test is named. Start by reviewing existing SwiftLint rule implementations and rule tests to find the appropriate entry points, then verify that selector- and block-based addObserver calls are flagged, post calls are not, and the rule is opt-in.