[objective_c] Collections support for NS[Mutable]Dictionary and NS[Mutable]Set

[liamappelbe]

NSDictionary and NSMutableDictionary now mix in MapBase, and NSSet and NSMutableSet mix in SetBase. The immutable variants just throw if the mutating methods are called.

The implementation is mostly straightforward. To make iteration easier, I also changed NSEnumerator to implement Iterator. That way, we can use the existing [key/object]Enumerator() methods to iterate the collections without having to add another layer of wrapping.

The one quirk is that NS[Mutable]Dictionary's keys must implement the NSCopying protocol, which means that on the Dart side we have a bunch of NSCopying objects in the API, instead of pure ObjCObjectBase. But the ObjC API isn't very type safe, so there are still a bunch of places where it uses ObjCObjectBase for the keys. So I had to do some castFrom calls, and add a _NSDictionaryKeyIterator that wraps the NSEnumerator and casts the elements as it's iterating.

Also, it turns out if a class extends Base and mixes in Mixin, the methods from Mixin will override any matching methods from Base. This means some of the methods on NSMutableArray were falling back to the slow versions on ListMixin, rather than using the fast versions from NSArray. So I had to duplicate some methods. Same logic applies to NSMutableDictionary and NSMutableSet.

Fixes #1444
Part of #1443

[github-actions]

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Non-Breaking	7.1.0	7.2.0-wip	7.2.0	✔️

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart