[WIP] Swift style KVO pattern

[liamappelbe]

Usage:

1. Implement the Observer protocol
2. Call object.addObserver(observer, ...)
3. addObserver returns an Observation, which must be held for as long as you want to keep receiving updates
4. The Observation object holds strong refs to the Observer and the observed object, keeping them alive
5. Call observation.remove(), or allow it to go out of scope and be GC'd, and the observations will stop

TODO: Observation should probably drop its references to the Observer and observed object when .remove() is called.

#1508

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.0.0	8.1.0-wip	9.0.0 Got "8.1.0-wip" expected >= "9.0.0" (breaking changes)	⚠️

This check can be disabled by tagging the PR with skip-breaking-check.

Changelog Entry ❗

Package	Changed Files
package:objective_c	pkgs/objective_c/lib/objective_c.dart pkgs/objective_c/lib/src/objective_c_bindings_exported.dart pkgs/objective_c/lib/src/objective_c_bindings_generated.dart pkgs/objective_c/lib/src/observer.dart

Changes to files need to be accounted for in their respective changelogs.

This check can be disabled by tagging the PR with skip-changelog-check.

API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols
objective_c	_FinalizablePointer _Version

This check can be disabled by tagging the PR with skip-leaking-check.

License Headers ✔️

// 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
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/native_doc_dartifier/example/native_doc_dartifier_example.dart
pkgs/native_doc_dartifier/lib/native_doc_dartifier.dart
pkgs/native_doc_dartifier/lib/src/native_doc_dartifier_base.dart
pkgs/native_doc_dartifier/test/native_doc_dartifier_test.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 86.987%. first build
when pulling 0aee008 on objc_kvo_swift
into 7f5bfa6 on main.