objective_c-v9.0.0

• package:objective_c is no longer experimental
• Use FFIgen 20.0.0
• Breaking change: The collection classes, NSArray, NSSet,
  NSDictionary, and their mutable counterparts, no longer directly implement
  the corresponding Dart collections. Instead they each have a .toDart()
  method that wraps the class in an adapter that implements the Dart collection.
  Note that this is a shallow conversion. For deep conversions, continue using
  toObjCObject and toDartObject.
• Breaking change: Rename the internal C types ObjCObject and
  ObjCProtocol to ObjCObjectImpl and ObjCProtocolImpl respectively.
• Breaking change: Rename the internal Dart types ObjCObjectBase and
  ObjCProtocolBase to ObjObject and ObjCProtocol respectively.
• Fix missing NSNumber category includes in iOS and macOS objective_c.m
  files.
• Add NSBundle and NSNull to the bindings.
• Add autoReleasePool function.
• Fix a bug where
  NSMutableDictionary.of returned a NSDictionary.
• Add NSErrorException class.