Fix NSAttributedString implementation in Foundation

[CKegel]

The current NSAttributedString implementation results in many compilation errors regarding unimplemented methods. I believe the implementation should be corrected to match the principles in the following Apple Developer documents: https://developer.apple.com/library/archive/documentation/LegacyTechnologies/WebObjects/WebObjects_3.5/Reference/Frameworks/ObjC/Foundation/Classes/NSAttributedStrngClstr/Description.html#//apple_ref/occ/cl/NSAttributedString https://developer.apple.com/library/archive/documentation/General/Conceptual/CocoaEncyclopedia/ClassClusters/ClassClusters.html#//apple_ref/doc/uid/TP40010810-CH4-SW1